Update build config

* Update to Gradle 1.12 and Android plugin 0.12, plus necessary tweaks in `build.gradle` to make it work. 
* Rename folders: now the library is in `library` and the demo app in `demo`
* Remove docs folder (it was there because it used to be referenced from the README but now it's not longer needed) 
* Update `README` and `.gitignore`