Add support for lists of fixtures

We already had the ability to define a bunch of fixtures as a list that
would then be accessible as fixture_name_<index>. This adds the
functionality to be able to get the whole list as fixture_name. That way
we can define one-to-many relationships really easily with this list.
Repair test docstrings

Charles reallly reallly likes docstrings that start with verbs. Now all
my test doctrings are beginning with "Assert" yey!
Add docs about lists of fixtures

 - Add to the "file format" docs information about defining lists of
   fixtures.
Fix typo in file format docs for fixture lists
Merge pull request #9 from uber/lists-of-fixtures

Add support for lists of fixtures