Added Gradle build support

Hi Nicholas,

I know you've kind of abandoned development of DataDroid but, as I migrated a project of mine on Android Studio, I had the need to have it build with Gradle. This pull request is just to add the build.gradle file to allow building the library with Gradle (and a .gitignore to exclude Gradle-built files from VCS).

It's then dead simple to include DataDroid in a Gradle project. You just have to clone it as a submodule (eg. in `{projectRoot}/submodules/datadroid`), reference it in the `settings.gradle` file:

``` gradle
include ':yourApp', ':submodules:datadroid:DataDroid'
```

And then add the dependency to your app's `build.gradle`:

``` gradle
...
dependencies {
    compile project(':submods:datadroid:DataDroid')
}
...
```