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.

blender_cloud/wheels/__init__.py

@@ -0,0 +1,41 @@
+"""External dependencies loader."""
+
+import glob
+import os.path
+import sys
+import logging
+
+my_dir = os.path.join(os.path.dirname(__file__))
+log = logging.getLogger(__name__)
+
+
+def load_wheel(module_name, fname_prefix):
+    """Loads a wheel from 'fname_prefix*.whl', unless the named module can be imported.
+
+    This allows us to use system-installed packages before falling back to the shipped wheels.
+    This is useful for development, less so for deployment.
+    """
+
+    try:
+        module = __import__(module_name)
+    except ImportError:
+        pass
+    else:
+        log.debug('Was able to load %s from %s, no need to load wheel %s',
+                  module_name, module.__file__, fname_prefix)
+        return
+
+    path_pattern = os.path.join(my_dir, '%s*.whl' % fname_prefix)
+    wheels = glob.glob(path_pattern)
+    if not wheels:
+        raise RuntimeError('Unable to find wheel at %r' % path_pattern)
+
+    sys.path.append(wheels[0])
+    module = __import__(module_name)
+    log.debug('Loaded %s from %s', module_name, module.__file__)
+
+
+def load_wheels():
+    load_wheel('lockfile', 'lockfile')
+    load_wheel('cachecontrol', 'CacheControl')
+    load_wheel('pillarsdk', 'pillar_sdk')