Add support for adding the library as a gradle dependency.

[sangaline]

The main change here is restructuring the repo so that the library is in the root of the project and the demo app is in a subdirectory. This allows the repo to be used with jitpack.io so that it can be added easily to Android Studio projects as a gradle dependency. Information about how to do this was also added to the README. Additionally, the gradle and target SDK versions were changed to use newer releases.

This addresses issue #5.