make the git revision visible from within pele

setup.py identifies the git revision and saves it in a created file
pele/version.py

the git revision can be found by

    import pele.version
    print pele.version.git_revision
Merge branch 'master' of https://github.com/pele-python/pele into cell_lists
Merge branch 'cell_lists' of https://github.com/smcantab/pele into cell_lists