Complete rewrite of the hook lib. [closes #3 #4]

* FIX: Run more than one script per hook. Now for real.
* Simplified DSL. Just pass the name of the scripts.
* To run scripts in background, pass `:&` as a last option, like when you use `&` in bash.
* Output background processes to `hooks/log`.
* Put all the scripts in `hooks/bin`. Much more simpler structure.