Add further gck commands

This adds the following gck commands:

• help: used to display usage information
• c: alias for gck console
• db: alias for gck dbconsole
• rake: allows passing arguments easily to rake tasks
• do and run: running commands, the latter attached to a tty

Why this is an improvement:

These are little things, but they enable things like:

alias be="gck bundle exec"

And then:

be rspec spec/models/project_spec.rb:123

Instead of:

gck command COMMAND="bundle exec rspec spec/models/project_spec.rb:123"

Note the lack of need for quotes or whitespace handling - this means it plays nicely with xargs, etc.

I have particularly valued gck c and gck db as aliases for the longer commands.