Project updates

[friederbluemle]

This PR contains a bunch of general project cleanups and dependency cleanups.

The project now uses the latest Android Gradle plugin, and Gradle wrapper. It was updated to target SDK 25 - Note that because of this update, minSdkLevel had to be raised to 9 (it was at 8 before). Please let me know if you prefer to split this PR up into multiple PRs.

Once the target is switched to 26, minSdkLevel has to be raised all the way to 14! Let me know if you prefer to do this now, or do this at a later point (after you have published at least one update of the library). I have a local branch ready, and can force push to this PR.

The project is fully compatible with the latest stable version of Android Studio (2.3.3).

_{^{Please use a normal GitHub merge (i.e. NO rebase/squash merge) to merge the commit(s) in this pull request. The changes are broken up into meaningful, atomic commits, and my branch should already be up-to-date with the latest target branch. If it isn't, or if you'd like me to combine something, please let me know, and I will make the necessary updates as soon as possible. Thank you!}}