In project_settings use local_vcs_root, not vcs_root

So that the vcs_root in fablib is set properly.  Also adjust tasklib to
use local_vcs_root.
fablib: get the proper local tasks bin

using the DEPLOYDIR environment variable
ve_mgr: add a clean_ve() method

that deletes the virtualenv.  Might be used by fab at some point.
tasklib: remove virtualenv management

as tasklib will be in the virtualenv.  So bootstrap (sometimes via
fabric) will need to manage the virtualenv.
Update README a little
In fab.py and tasks.py, if in an active virtualenv then use it
Do git submodule update in bootstrap process

Otherwise we'll have problems when pip requirements refers to a local
directory that is actually a submodule.