Remove Mailbox from credentials tests
Merge branch 'misc-changes' into getevents-paging
Create EventIterator object.

Also, define current, deprecated interface to events in terms of the
iterator.  No functionality removed.
Implement iterator navigation.

To discover when you're at the beginning or end of the navigation set,
check for len(ei.Events())==0 after having called GetNext() or
GetPrevious().
Remove original GetEvents() API.

Replaced with EventIterator-based API.  TODO: godocs.
Replace GetEvents with NewEventIterator
Merge pull request #18 from mailgun/getevents-paging

Getevents paging.  Optimistically merging.