Cleanup IDE files, update build dependencies to latest

[micolous]

This pull request comprises two parts:

1. Delete all the IDE files. Because of multiple issues with IDEA (which they have previously claimed to have fixed), it can cause ugly conflicts with these files checked in to version control.

This is even evident in the other pull request, #3.

I've imported this as a Gradle project (sans IDE files), and this is sufficient to bootstrap the project in to being workable with IDEA... once the app.properties file is supplied.

2. Update build dependencies to use SDK 23 and build-tools 23.0.2. This was upgradable cleanly without any code changes needed.