Add API to re-use request builders

[sjudd]

Fixes #182, and is substantial progress toward #185.

The basic idea is that after these changes you can now call load() on requests builders at any point:

requestBuilder = Glide.with(fragment)
    .from(String.class)
    // apply options here
    ;

requestBuilder.load(url).into(view1)
requestBuilder.load(url2).into(view2)

[sjudd]

I pushed the change to Cloneable, it's definitely cleaner than manually copying the instance variables for this limited use case. Thanks for looking!

[TWiStErRob]

This modification is a huge step, because now you can pass down a GenericRequestBuilder to Adapters instead of passing down the Fragment to create it. The "how" of the loading is not known to the adapter.

Does trying to be backwards compatible making it harder to make such a change?

[sjudd]

Aside from our discussion in the SVG app, being backwards compatible wasn't really an issue here. I have to retain the .load() APIs, but those are convenient anyway if you want to do a one-off load and otherwise nothing much was affected other than the Animations.

It may even be possible to add an API like the one we're discussing for the SVG sample without breaking anything.