ENH: Replicating command line fif handling/formatting

This branch changes the writing of inverse and covariance fif files to replicate the fif file output of the command line tools because I am having problems using python generated inverse fifs in matlab. Mne-python was evaluated by reading in an inverse file from the sample data, rewriting it to a fif, and checking fixing inconsistencies. 

The write_inverse_operator function was reorganized so that output lined up with the command line version.  Code to handle units (Am, Am/m^2, ...) was added to both the read_inverse_operator_and write_inverse_operator. Code to optionally leave out bad channels/projection information, which are not in command line fif output, was added.

Passed nosetests:
mne/tests/test_cov.py
mne/minimum_norm/tests/test_inverse.py