use Gradle Wrapper for build

This can only be built with gradle 1.10. Having (current) gradle 2.1 leads to this:
```
$ gradle
FAILURE: Build failed with an exception.
* Where:
Build file '/home/jonnyjd/git/_android/controldlna/build.gradle' line: 12
* What went wrong:
A problem occurred evaluating root project 'controldlna'.
> Failed to apply plugin [id 'android']
   > Gradle version 1.10 is required. Current version is 2.1. If using the gradle wrapper, try editing the distributionUrl in /home/jonnyjd/git/_android/controldlna/gradle/wrapper/gradle-wrapper.properties to gradle-1.10-all.zip
```

There is a solution to this, coming from gradle, the [Gradle Wrapper](http://www.gradle.org/docs/current/userguide/gradle_wrapper.html). It is suggested to add a `graddlew` script to the repository that gets and uses the appropriate gradle version.

This is what I did.
The only change by me is in build.gradle, the rest was done by running `gradle wrapper`.
I did have to install gradle 1.10 for that step, but I don't need to switch back and forth again. (in fact no gradle installation is necessary anymore)

This works on *NIX and Windows.