Add basic executable

This commit adds the example `pixel.php` as `vendor/bin/pixeler`. To use it the recommended method would be to run `composer global require 'lastguest/pixeler' 'dev-master'` then add `$HOME/.composer/vendor/bin/` to `$PATH`. Users could then run `pixeler -f file.jpg -w 0.4` etc from anywhere.

Also fixes minor mistake in README example