Custom Business Month
This extends the `offsets.py` module with a `CBMonthEnd` based on `CustomBusinessDay`. Tests are found in `test_offsets.py`. 

A few small points still open: 
- Implement CBMonthBegin
- Improve speed. Main speed drag is `DateOffset.onOffset`. 

Also some of the tests are not passing, but the errors seem unrelated. 
e.g.

```
======================================================================
ERROR: test_pickle (pandas.tseries.tests.test_timeseries.TestTimeSeries)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/benjamin/workspace/pandas/pandas/tseries/tests/test_timeseries.py", line 1827, in test_pickle
    p = pick.loads(pick.dumps(NaT))
  File "/usr/lib/python2.7/pickle.py", line 1382, in loads
    return Unpickler(file).load()
  File "/usr/lib/python2.7/pickle.py", line 858, in load
    dispatch[key](self)
  File "/usr/lib/python2.7/pickle.py", line 1133, in load_reduce
    value = func(*args)
TypeError: __new__() takes exactly one argument (2 given)
```

did u rebuild extensions? this was just added in cython

python setup.py build_ext --inplace
 Ok that was part of it. Apparently Travis is on numpy 1.6 so the CBMonth (requires numpy 1.7) tests are failing. 
 their are multiple versions of numpy tested with Travis 
look at the very end of the build u can see the print_versions
 Ah good hint, found the problem, will send an update later tonight. 
 I added `CBMonthBegin` and squashed the changes. Anything else to do? 
 pls rebase (u need to incorporate #6873 the apply_nat)
 How should I reference the enhancement in `v0.14.0.txt`? :issue:`6866` ? 
 yep (when no associated issue the pr number works too)
 Rebased, added a small section to the docs. Anything else? 
 can u do the Apply_nat decorator like other apply? it's just more consistent that way

@jorisvandenbossche @TomAugspurger 

comments?
 @bjonen can you rebase?
 done
 thank you sir!
 FYI, had to do this little patch https://github.com/pydata/pandas/commit/a189d5d0775e46cb8fc2a68955c85e499ed04e6c

was failing on windows 2.6 for some reason (and not sure why didn't fail on linux)
 Ok, thanks! 
