Added Android Plugin support. Pulls in the Main Source.

This uses the Main `sourceSet` of the Android plugins (Android/Library) they both inherit from the BasePlugin now.

This _shouldn't_ be nessary, but setting https://github.com/kt3k/coveralls-gradle-plugin/blob/master/src/main/groovy/org/kt3k/gradle/plugin/CoverallsPluginExtension.groovy#L26 has no effect on the `project.extentions.coveralls.sourceSets` property.

If you fix that then, this becomes slightly obsolete.