Gradle support

Finally added gradle support for this lib. thanks @wuyingren for you initial commit.

Thanks @wodka for the missing source folder hint in the `build.gradle`

The library is working. The sample is working.


open TODOs:

- [ ] simple Eclipse import for the library
- [ ] quick start with sample and lib in Eclipse

- [ ] simple Android Studio import for the library
- [ ] quick start with sample and lib in Android Studio