more efficient path parsing

* no call to sed
* use of the "\w" for bash default value when path shorting is desactivated
my way of shorting path was way slower than the one originally implemented

redoing the dirtrim feature though
no systematic setting of PROMPT_DIRTRIM
adding comments and documentation
optimisation : less operation
changing my mail
Merge pull request #89 from jaesivsm/feature/dirtrim

Feature/dirtrim