Refactor how Context references are held.

This also adds support for Ion.with(Fragment), and respects fragment lifecycle.
java doc for new methods
Update ion examples.
Provide access to caches/stores
Merge branch 'master' of https://github.com/koush/ion

# By Koushik Dutta
# Via Koushik Dutta
* 'master' of https://github.com/koush/ion:
  Provide access to caches/stores
  Update ion examples.
  java doc for new methods
  Refactor how Context references are held.