WIP:  Read Polhemus Montage

Read Montage from Polhemus Point files (following up on https://github.com/mne-tools/mne-python/issues/1703#issuecomment-67361977)

To do:
- [x] allow point names to be specified in a file
- [ ] TESTS
- [ ] update DOCs

Questions:
 - ~~Do you prefer leaving this in `io.meas_info` or would you rather move it to a dedicated `io.polhemus` module?~~
 - ~~Can it be a problem to have superfluous points in the montage? (Should I drop Nasion, LPA and RPA after using them for the transform or add them to the Montage?)~~
 - `Montage.__init__` does not check its inputs, is it only for internal use so we can assume they're safe?

that's btw. one reason why format is nicer. It does not confound syntax with values