add initial command-line interface

You can see commands by running "copay getCommands"

Types are enforced - right now only strings work. This is so that types can be
coerced from the command-line correctly. Number of arguments are also enforced.
In the future you will be able to pass in JSON objects as arguments and they
will be translated to real objects.
Merge pull request #39 from ryanxcharles/feature/command-line

WIP: command-line interface