Use YAML for Configuration and Rush Files

Drush 6 & 7 will parse YAML config files.

Ideally, I'd like to have Rush use a single YAML file (rather than a job directory) to define parameters and rush operations.

Here are some things that will need to be worked on:

Define the YAML format for Rush (including params and rush operations)
Should include ability to define by environment (see https://gist.github.com/seanbuscay/8364715)
Should allow for the use of constants (see http://librosweb.es/symfony_1_0_en/chapter_5/tips_for_getting_more_from_...)
Ability to use Aliases -> http://stackoverflow.com/a/10593087