Switched caching to CacheControl + pillar module usable without Blender

Making the blender_cloud.pillar and blender_cloud.cache modules usable
without Blender required some moving of the code, from __init__.py to
blender.py.

CacheControl requires the lockfile package, which increases the number
of bundled wheel files to 3. Those are now managed by
blender_cloud.wheels.load_wheels(). The wheels are only loaded if the
Python installation doesn't yet contain the required packages. This allows
development with virtualenv-installed packages, debugging in the IDE, etc.

requirements.txt

@@ -0,0 +1,2 @@
+CacheControl==0.11.6
+lockfile==0.12.2