Huxley

* Support concurrent runs (`-c <num>`)
* Use Pillow instead of PIL

* Fix handling of dependencies
* Fix new versions of Firefox by upgrading Selenium

* Expose `--save-diff` command-line option instead of requiring an external image diff tool
* Expose `screensize` parameter in Huxleyfile
* Expose `--version` command-line option

* Initial open-source release

* Pre-beta