issue #96: a way to disable vcs depending on a list

redoing
forked from the wrong branch and couldn't add support for bazar
issue #95: no double call to _lp.*branch()

_lp_smart_mark is now called with an argument specifying if a CVS mark
is needed
noticeable execution speed improvement
refactorisation for fixes concerning issue 95/96
do not recalculate some vars if the $PWD hasn't changed

vars that will not be recalculated whithout path change :
* LP_VCS_TYPE : the type of repository we're in (git / svn / nothing
* / ...)
* LP_PWD : the (maybe shortened) path
* LP_MARK : the prompt mark which depend on the type of repository
* we're in
* LP_PERM : the color of the colon indicating the permission for the
* current directory
modification of the documentation concerning the LP_VCS
fixing regression : lost the smart mark in vars renaming
Merge pull request #98 from jaesivsm/feature/vcs

Feature/vcs