Refs #252: Simplify handler loading

New setting RAPIDSMS_HANDLERS is a list of names of handler
classes to load. The previous settings are deprecated.
Merge branch 'develop' into 252-simplify-handler-loading

Conflicts:
	docs/releases/0.14.0.rst
Update version numbers in docs; use import_class instead of try_import
Start on tutorial part 2
Refs #252: Add tests, a little more doc
Merge branch '252-simplify-handler-loading' into tutorial

So I can assume the new RAPIDSMS_HANDLERS setting and talk
about that in the tutorial. Much simpler than the old
settings to explain.
Flesh out tutorial 2; still need to check that the examples work