Update offset prefix map
This should address issue #5382 

I tried setting up Travis but ~~can't tell if it's working.~~ didn't turn on the service hook before pushing. Locally I ran `nosetests pandas` but I don't know what to think of the results:

Base case - `master` (d0342aa79d2) 2013-10-27 20:26:02

```
Ran 4331 tests in 384.605s

FAILED (SKIP=318, failures=1)
```

Test case - `update-offset-prefix-map` branch (a2cc2303681)

```
Ran 2977 tests in 190.863s

FAILED (SKIP=156, errors=1064, failures=1)
```
 Weird that master has a failing test case. It appears that you introduced
some errors such that 1300 tests didn't run and another 1000 tests failed
in some way. Many tests create date indexes as part of their setup, which
may be the problem.

Also, Nano is conditionally added to the offset list based on numpy version.
 @patricktokeeffe and generally doing something based on the import, e.g. `__all__` is not a nice way way of doing things
 Ok. Looks like the only missing one then is `LastWeekOfMonth` and since it appears to be WIP (#4637) this PR is irrelevant. 
