Remove generated / local files from git

Remove and exclude the generated or local-only directories / files:

 - bin/
 - gen/
 - local.properties

Removing local.properties means that if you use ant to build an APK,
you may need to take additional steps to re-generate the
local.properties file. See additional comments in the README for
more details.

Signed-off-by: Jeff Godin <jgodin@tadl.org>