Add configuration files for Android Studio

- add gradle configuration files
- replaces most libraries with maven dependencies (except for
  SherlockNavigationDrawer)
Update gitignore for Android Studio
Add gradle wrapper
Use toxbee maven repo

This repository contains a gradle compatible version of
android-numberpicker.
Update building information

Add information on how to import the project to Android Studio.
Add missing gradle-wrapper files

These files were missing in commit 8e46c3e because of the .gitignore
config
Merge pull request #99 from d-sko/android_studio

Building with Android Studio