Use custom MasterChooser

[jubeira]

Apart from revamping the current MasterChooser, it would be nice to add a standard way of using a custom MasterChooser that could be integrated to a settings activity. One possible approach to accomplish this would be the following (taken from conversation in #239):

• Add a constructor receiving an intent for the activity to launch, and a custom request code.
• Launch the custom activity in startMasterChooser using the custom request code (the current MasterChooser will be launched with the default constructor).
• Finally, the code inside onActivityResult would be placed inside a callback that could be set with a public method. The default behavior wouldn't be modified, but the user could choose to perform different actions and even handle different results or request codes.

Any comments on this are welcome.

[jubeira]

I can start working on this BTW, I will submit a PR once it's ready and tested.