New offsets
These are the new offsets and frequency strings posted on the wiki.

I haven't included any deprecations because I'm not clear how this should work yet.

Quick question: one of your tests fails, 

datetime(2007, 1, 1) + MonthBegin(-1) is datetime(2007, 1, 1) and not datetime(2006, 12, 1) as you suggest in the test. I think it should indeed be datetime(2007,1,1) (b/c add with n<0 is rollforward and add). can you just confirm?
 NM, I say the typo that led to the behavior (in MonthBegin, it was =+ instead of +=.  Lovely)  Now all the tests pass.
