Gradle support added

Hello, I've forked a repository that was forked from yours that was starting to add support for gradle.

I've finished adding support for Gradle and made it create a Maven repository, so that it's possible to retrieve the dependency from the Web.

Currenctly, the person who wants to use your library would add to the build.gradle:

Repository:
maven { url "https://raw.github.com/kolavar/android-support-v4-preferencefragment/master/maven-repository/" }

Dependency:
compile ('com.android.support:support-v4-preferencefragment:1.0.0@aar'){
    exclude module: 'support-v4'
}