Use Click for the CLI

This replaces the existing `CLI` class with a `cli` function and uses the great [click](http://click.pocoo.org) library to deal with common CLI paradigms.

Some points to note:
* This allows the use of setuptools' `entry_points` to avoid having to ship a bin script and makes the project more installable.
* Writing to `stdout`/`stderr` has been been replaced with `click.echo()` which deals with various terminal issues and differences between python 2 & 3 when doing this.
* A minor change in behaviour - a keyboard interrupt now causes an exit code of `-1` since this is what click does by default.

This specifically scratches my itch for installing from PyPI using [pipsi](https://github.com/mitsuhiko/pipsi).