Move F-Droid project into subdir, keeping a root gradle project

Also improved .gitignore a tad
Merge branch 'gradle-structure'

As described on https://f-droid.org/forums/topic/gradle-directory-structure/.

This will keep things a bit weird with git for a bit, but you can always use
--follow to make it take renames into account for the commit history.

I tweaked both the ant and gradle build setups and both of them work in the
new structure. And gradle is much faster since e.g. now an 'assemble' inside
F-Droid assembles only the client, without including all the subprojects as it
happens in master.
Bump to 0.77-test