Add __version__ to package according to PEP396

* Adds __version__ attribute to flask_registry according to PEP396.

* Derives setup.py and Sphinx version argument from version module.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Add BlueprintAutoDiscoveryRegistry

* Adds registry for discovery and loading of Flask Blueprints.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Differentiate between missing vs broken modules

* Ensures missing modules are not treated as errors, while exceptions
  raised by broken modules are properly reraised.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>