Update behavior for incoming SMS path.

1) On KitKat, unencrypted SMS messages are never stored in
   TextSecure unless it is set as the system-wide default.

2) On KitKat, if TextSecure is set as the system-wide default,
   provide an option to change the default to a different app.

3) Don't store the TextSecure challenge on KitKat+ devices.
Better support for local storage with passphrase disabled.

1) Never get into a state where messages aren't displayed
   unencrypted.

2) Fix bug where locked DB required launching twice.
make bithub price update // FREEBIE

thanks lilia
Add contributing.md // FREEBIE

This file is recognized by github and presented to users who are trying
to open issues and submit pull requests.

Hopefully this more prominent placement will help deter people from
opening issues/prs for translation fixes.
Merge pull request #921 from liliakai/master

Add contributing.md // FREEBIE