Migrates project to Gradle and removes 3rd party library from code

This converts the project over to Gradle so it can easily be imported into Android Studio. It also creates a dependency to org.jraf:android-switch-backport:1.3.1 as opposed to adding the code directly into source. This allows for other libraries that use android-switch-backport to work alongside android-betterpickers. 

Now, to add the project as a submodule in Android studio all you have to do is
`repositories {
    maven {
        url "http://JRAF.org/static/maven/2"
    }
}
dependencies {
    compile fileTree(dir: 'libs', include: '*.jar')
    compile project(':android-betterpickers:library')
    compile 'org.jraf:android-switch-backport:1.3.1'
}`

or the simple 

`compile ("com.doomonafireball.betterpickers:library:1.5.2") {
    exclude group: 'com.android.support', module: 'support-v4'
    exclude group: 'org.jraf' module: 'android-switch-backport'
}` 

once the changes are pushed up to maven.