Add Config and Client classes

Currently, this will only add a `.tusker` file to the root directory of a
Mac/Linux type machine. Windows is going to be another piece to add later,
I presume.

*Major* help derived from a config class built by @kytrinyx,
from the original CLI for [http://exercism.io](http://exercism.io)

wip

Add Config and Client classes
Add default task to Rakefile, and Console task

Default task is set to run specs.
Add ability to list notebooks, and add a token, via Tusker
Add usage/caveats to README