Add to_fwf support
Right now pandas can read fwf but there is no `to_fwf`.
From [SO](http://stackoverflow.com/questions/16490261/python-pandas-write-dataframe-to-fixed-width-file-to-fwf).

This output format seems similar to what [`numpy.savetxt`](http://docs.scipy.org/doc/numpy/reference/generated/numpy.savetxt.html) handles.
