Allow setting the code version for android

[waltjones]

Fixes: #106

To enable Proguard support, allows setting the code version in AndroidManifest.xml:

<application>
  <!-- ... -->

  <meta-data android:name="com.rollbar.android.CODE_VERSION" android:value="1.0.0" />
</application>

#106 suggests adding to the arguments for init(), but this doesn't scale well as more config options are needed in the future, and method overloading for init() is unmanageable when all the args are the same type (String).