Added Gradle Support

### If you have gradle installed
from the `/library` folder, in terminal run: `gradle clean build`

### If you want to use the gradle wrapper
from the `/library` folder, in terminal run: `./gradlew clean build`

The final .aar will be created in the `/library/build/outputs/aar/` folder


### Gradle Wrapper
The gradle wrapper allows you to run the build without having to install the gradle binary on your machine.  This works great if you must build on a machine but do not have permissions to install anything.


### Modifications made to AndroidManifest.xml
I made some slight modifications to the `<application>` node in the manifest.  Library projects shouldn't include any attributes in the `<application>` node.  Specifically, the `android:allowBackup` attribute can cause conflicts when including in other projects.