Adds FormattedItem class

This changes how the CLIRunnables define formatted values in a way that will perserve
the raw value. The signature is this: FormattedItem(raw, formatted). Along with this is
a helper that converts raw megabyte values to gigabytes.
Adds listing format helper.

* Usage: listing(speeds, separator=',')
* Returning a list from execute() in CLIRunnable will now print out
  each item separated by new line.
Merge pull request #39 from sudorandom/better_formatting

Adds FormattedItem class