Fix module doc-string
Split out enterprise support in github3.login

Add github3.enterprise_login
Rename all the iter_(*) methods to \1

- Above regular expression example:

iter_all_users -> all_users
Remove bad API for (subscribing to|ignoring) repo notifications
Remove legacy/deprecated watching API
Update HISTORY.rst with changes
Dedent the table

It probably should not have been indented in the first place
Rename github3.orgs to github3.organizations
Rework the public functional API
Add NullObject
Add tests around new methods

- Add cassettes for new methods
Start rewriting unit tests properly
Fix failing unit test

We were incorrectly testing github3.enterprise_login
Return what I really expect __iter__ to do

What convinces me of this is what happens if you do `list(NullObject())`. The
list should be empty but instead you received a single-element list with the
NullObject instance in it. That's just wrong.
Test NullObject
Start migrating old tests
Add a bunch of urls and url templates we were missing
Use pep257 for Subscription's docstring
Assert condition about correct attribute
Remove tests around removed legacy API