ENH: Better method for getting user home folder

mne.utils module attempts to find the user home folder by searching for
the environment variable HOME; however, this is not necessarily
available depending on how the Python program was launched (e.g. via
Upstart).  Switched to a more robust method for identifying the home
folder.
Merge pull request #980 from bburan-galenea/bburan-galenea/env-var-fix

ENH: Better method for getting user home folder