ENH: Don't require MNE configuration file

Fixes #1166

If the application is running in a daemon (e.g. uWSGI), the home
directory may not be writeable (for security purposes).  In such a
situation, `mne.utils.get_config_file` will raise an OSError.  Intercept
this error and return the default value for the configuration variable
unless `raise_error` is True (in which the original behavior is retained
and the user gets an error message indicating they need to create a
configuration file or set an environment variable).