Restore post object when returning to destroyed EditPostActivity. Fixes #422.

 * Blog and Post models now implement Serializable
 * In onSaveInstanceState of EditPostActivity, the post and original post objects are stored in the bundle.
 * In onCreate() of EditPostActivity, if the savedInstanceState is not null, then the two post objects are restored.
* Added serialVersionUID to Blog and Post model.
* Fix compiler error in Post.equals() method.
* Catch ClassCastException when loading posts from savedInstanceState, just in case.
Merge pull request #423 from wordpress-mobile/issue/422

Restore post object when returning to destroyed EditPostActivity. Fixes ...