ENH add_dig_points, SetChannelMixin, reorganization

This is WIP.

Adds function apply_dig_points to apply hpi, cardinal, and dig points to the Info.

Remove unnecessary import functions for np.loadtxt.

Needs test. Not ready for review
you can use the hpts that is defined in the manual and there is an example in the the montage module. it's a standard format supported by MNE.
this is why I initially had the function the way it was designed so it can be specified another. also it was a point I was arguing for in the montage but it is right that the fiducials should be treated separate from the electrode.

if you want to import brainvision dig points, you just need to do the following:
raw = read_raw_brainvision(fname, montage='your_brainvision_file.hpts')
hsp = mne.io.meas_info.read_dig_points('hsp_file.txt')
d = mne.io.meas_info.read_dig_points('dig_points_file.txt')
raw.info['dig'] = mne.io.meas_info.add_dig_points(nasion=d[0], lpa=d[1], rpa=d[2], hpi=d[3:], hsp=hsp)