Add Bazaar support

The support is still quite limited:
  - green if not modified
  - red   if modified (shelved changes are currently considered as changes)
  - Current revno is displayed

TODO:
  - Since bzr is slow to start, refactor liquidprompt to not invoke bzr twice (smarttag and bzr_prompt)
  - Display not pushed changes
Merge pull request #94 from cykl/develop

Add Bazaar support
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
Correction 'update_terminal_cwd error' on Mac OSX (Mountain) Lion systems
Merge branch 'develop' of github.com:nojhan/liquidprompt into develop
Merge branch 'master' of https://github.com/dloureiro/liquidprompt into develop
fixing regression : lost the smart mark in vars renaming
Merge pull request #98 from jaesivsm/feature/vcs

Feature/vcs
cleaning: no useless color definition

* no color definition in PS1 setting, if the functionnalitie is desactivated there is no need to define a color
* no NO_COL in the begining of functionnalitie output, the previous one is supposed to end with a clean color (NO_COL)
do not recalculate darwin kernel version at each prompt set
it doesn't seem that LP_USER need to be defined at each prompt set
optimisation: use built-in time vars \t and %*
Update the demo screenshot
Merge branch 'develop' of github.com:nojhan/liquidprompt into develop
bugfix #99: bad quotes around virtualenv name
removing another useless NO_COL