Refs #252: Simplify handler loading

New setting RAPIDSMS_HANDLERS is a list of names of handler
classes to load. The previous settings are deprecated.
Refs #360: Add contributing example to docs
Refs #360: Mention new doc page in release notes
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
Sketch outline of tutorial
Clearly mark the tutorial as a draft so we can put it on RTFD for comments
First pass at tutorial part 1
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
Finish up tutorial02
Start on tutorial03
Improve registration tests, doc

* Modify the registration app tests so they'll continue to work after
  we turn off the old handler loading behavior.
* Update the doc for the echo app to use the new handlers setting.
Add note about application design; a few other tweaks
Start tutorial part 4 - introduce Tropo
More on Tropo
Write more about using Tropo
Add some explanation of Contacts, Connections, and identities