Add is_loading and is_saving values to cereal::{Input,Output}Archive

This provides compatibility for asymmetric serialize routines when transitioning from boost serialization and associated archive types.

See: USCiLab#360

Example:
```
    template <typename Archive>
    void serialize(Archive &ar, const std::uint32_t version)
    {
        std::int16_t tmp = foo;
        if(Archive::is_loading::value)
        {
            ar & tmp;
            foo = tmp; // For illustration only
        }
        else
        {
            tmp = foo; // For illustration only
            ar & tmp;
        }
        ar & foo;
    }
```

include/cereal/cereal.hpp

@@ -255,6 +255,20 @@ namespace cereal
           a large project from Boost to cereal.  The preferred interface for cereal is using operator(). */
       //! @{
 
+      //! Indicates this archive is not intended for loading
+      /*! This ensures compatibility with boost archive types.  If you are transitioning
+          from boost, you can check this value within a member or external serialize function
+          (i.e., Archive::is_loading::value) to disable behavior specific to loading, until 
+          you can transition to split save/load or save_minimal/load_minimal functions */
+      using is_loading = std::false_type;
+
+      //! Indicates this archive is intended for saving
+      /*! This ensures compatibility with boost archive types.  If you are transitioning
+          from boost, you can check this value within a member or external serialize function
+          (i.e., Archive::is_saving::value) to enable behavior specific to loading, until 
+          you can transition to split save/load or save_minimal/load_minimal functions */
+      using is_saving = std::true_type;
+
       //! Serializes passed in data
       /*! This is a boost compatability layer and is not the preferred way of using
           cereal.  If you are transitioning from boost, use this until you can
@@ -611,6 +625,20 @@ namespace cereal
           a large project from Boost to cereal.  The preferred interface for cereal is using operator(). */
       //! @{
 
+      //! Indicates this archive is intended for loading
+      /*! This ensures compatibility with boost archive types.  If you are transitioning
+          from boost, you can check this value within a member or external serialize function
+          (i.e., Archive::is_loading::value) to enable behavior specific to loading, until 
+          you can transition to split save/load or save_minimal/load_minimal functions */
+.      using is_loading = std::true_type;
+
+      //! Indicates this archive is not intended for saving
+      /*! This ensures compatibility with boost archive types.  If you are transitioning
+          from boost, you can check this value within a member or external serialize function
+          (i.e., Archive::is_saving::value) to disable behavior specific to loading, until 
+          you can transition to split save/load or save_minimal/load_minimal functions */
+      using is_saving = std::false_type;
+
       //! Serializes passed in data
       /*! This is a boost compatability layer and is not the preferred way of using
           cereal.  If you are transitioning from boost, use this until you can