Use go-cli, add version/serve subcommands

[carlosms]

Fix #244.

Using go-cli, now the command looks like this:

$ ./gitbase-web version
gitbase-web version dev-c9b1196-dirty build 10-01-2018_17_06_22

$ ./gitbase-web serve -h
Usage:
  gitbase-web [OPTIONS] serve [serve-OPTIONS]

starts serving the application

Help Options:
  -h, --help              Show this help message

[serve command options]

...

[smola]

Great. I tried finding a way to add default commands cleanly (so that we can have no subcommands and --version) to go-cli, but didn't find it yet.

[carlosms]

Yeah, I'd prefer to skip the subcommand. But I guess it also makes sense for the sake of consistency if all our bins have binname version, for example.