A couple of fixes, update API target to Android 4.4

This is the full list of changes:
* Removed the `+` from the layout files' id fields (they're already defined, you shouldn't have the plus in there)
* Set target API level to 19 (Android 4.4 KitKat) in the library manifest and in the Gradle build file (for both  `targetSdkVersion` and `compileSdkVersion`)
* Tweaks to the Gradle settings: use the newer 0.6.x Android plugin and version 19 build tools; declare the `defaultConfig` to match what is declared in the Manifest
* Tweaked a bit the `Switch` class code: now it uses the original API calls that were substituted in the backporting process if running on a platform version that supports them; removed some code in `onMeasure` that wasn't actually used for anything

The Gradle changes should be fine as they're pretty basic but I haven't had the chance to test them as much as I'd like. I am not confident with Maven, so that will need someone else to work on it.