Flags for GPM - go binary, the file to parse and disable -u on go get

Warning: This does break backward compatibility with of specifying the Godep file in $2, but this should be far more flexible in the long run, so I think this is worth it.

There are 3 new options, primarily to make working with GAE much easier.

1) -f The file to parse. Was required, simply because otherwise $2 could be a flag or the file, made things trickier.
2) -g What go binary to use. If you are using GAE, you will want to use the `goapp` command from your local GAE dev install.
3) -u Be able to manually turn off the -u option on the `go get` operation. This is just because GAE has libraries in it's GOROOT, and sometimes they conflict. It sucks, but this is a nicer workaround.

Keen to hear feedback, and let me know how I can help test / improve this.  It working for me locally in my project.