Update offset prefix map

[patricktokeeffe]

This should address issue #5382

[patricktokeeffe]

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 (d0342aa) 2013-10-27 20:26:02

Ran 4331 tests in 384.605s

FAILED (SKIP=318, failures=1)

Test case - update-offset-prefix-map branch (a2cc230)

Ran 2977 tests in 190.863s

FAILED (SKIP=156, errors=1064, failures=1)

[jtratner]

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.

[jreback]

@patricktokeeffe and generally doing something based on the import, e.g. __all__ is not a nice way way of doing things

[patricktokeeffe]

Ok. Looks like the only missing one then is LastWeekOfMonth and since it appears to be WIP (#4637) this PR is irrelevant.