Added support for outer join in DataFrame.update
Can you add a test and hook up travis-ci? thanks
 @dougvk you should rebase this on top of master first (an additional test case for the Exception was added here plus a change to error message).
 @dougvk need:
- tests
- hook up to travis
 whoops, completely forgot. will get on this
 @dougvk ? tests and pls hook up to travis
 Stalled.
