Independent builds and python support

This MR introduces various features:

1. This package is now built completely independently of the bob software stack (and bob.conda). It ships its own version of dependencies and can provide better support for python versions that are not necessarily supported (yet) by Bob or BEAT
2. As a bonus, you can now install this package alongside your conda base environment
3. An optimisation during the bootstrap process is now possible since we don't need to install a new environment for bdt using python-3.6, it can just install in the base environment itself.
4. This MR also makes git-clean a bit less verbose, on request form @amohammadi
5. Finally, it fixes #13 (closed), as reported by @heusch

bob/devtools/build.py

@@ -515,7 +515,7 @@ def base_build(server, intranet, recipe_dir, conda_build_config,
       python_version)
   if candidate is not None:
     logger.info('Skipping build for %s-%s-(py%s_?)%s for %s - exists ' \
-        'on channel', candidate[0], candidate[1], candidate[2], py_ver)
+        'on channel', candidate[0], candidate[1], candidate[2], py_ver, arch)
     return
 
   # if you get to this point, just builds the package