Better support for retrieving mapping.txt file

This pull request is mainly to update how the mapping file is retrieved to use the new API added in Android Gradle Plugin 0.13.0. I took the opportunity to also tweak a couple of other things with groovy syntax (15e03d3 and e61ec06) and changes in the plugin API (b2e9687). This came about due to the android gradle plugin recently changing where the mapping.txt file was stored (I commented about it in #37). 

I tested this locally with our project and it worked fine, but you may want to test this with iOS and non-android-gradle-plugin projects. The implementation change functionally should only apply to Android projects with the gradle plugin.

One potential caveat of this is that the minimum supported gradle plugin version would now be 0.13.0+ (due to the new API). This is what is supported in the latest version of Android Studio Beta (0.8.14), and the plugin update itself has been available since September 18th. If you for some reason don't want to raise this minimum, 7d8c2ee alone should cover the new path change (requires a minor tweak to handle the event that there is no flavor).

Let me know what you think! I annotated some of the code in this PR to explain a few of the nitty gritty bits. We've been using this plugin awhile, and it's been of amazing use to us.
Should that be configurable in the `hockeyapp` config section in the `build.gradle`?
If yes it should be moved to the `HockeyAppPluginExtension`.