Gradle as only build system + k9mail library

We decided to use [Gradle as only build system](https://groups.google.com/forum/#!topic/k-9-dev/0ErED-Ia338). So this PR removes support for Ant and Eclipse builds. Also, the directory structure was changed to the Gradle default, so we don't have to manually specify source sets.

In addition to that, the changes from PR #521 have been replicated to extract the 'mail' package into a library project.