Added maven support to dev branch solving issue #89

This simplifies integrating mopub-client into maven android project. I myself also have a maven based project, so i needed to make mopub-client into a maven library.

It would be nice to add this, as it will result in more adoption of your SDK.

Maven artifact name (feel free to change it):

<groupId>com.mopub.mobileads</groupId>
<artifactId>mopub-android-sdk</artifactId>
<version>1.0-SNAPSHOT</version>