Fixed #210, providing a simple Ruby API over SharedPreferences.

More info on the Ruby docs:
https://github.com/calabash/calabash-android/blob/master/documentation/ruby_api.md
Implemented get, set and clear preferences as Android instrumentation so they
can be called before or after an application or activity starts or finishes.

This is decided internally in operation based on whether the app is running already
or not.
Merge branch 'issue-210' of git://github.com/ustwo/calabash-android into ustwo-issue-210