Better phpbrewrc support. Goodbye cd hooks.

As promised in #224, here is another bash script, with cd hooks, that solves #223 in a more elegant way.

Now I'm using a DEBUG trap which is completely unobtrusive and won't break rvm or any other script that overwrites builtin bash functions (yuck rvm).

TODO:
- support for zsh

PS: #224 was merged by accident, sorry.