Support numpy 1.8
Numpy 1.8 was released recently - https://github.com/numpy/numpy/blob/master/doc/release/1.8.0-notes.rst

We should probably support it and add/change a Travis build for it. Additionally, might be interesting to benchmark numpy's nanmean, nanvar and nanstd against pandas versions...

numpy also notes that they are planning to make more datetime changes in 1.9 - whee!
