geogig: replaces geogit due to renaming of project

See: http://geogig.org/

Also, adds `JAVA_HOME` wrapper scripts and `geogig-gateway` server.
Can this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!