[MAINT] Give intersphinx links explicit roles

[tsbinns]

@larsoner pointed out in #220 that we should be explicit about the roles of intersphinx links since behaviour depends on default_role and could silently fail, e.g.

:class:`mne.Epochs`

rather than

`mne.Epochs`

I've been quite lazy with this, e.g. in the decoding module. Reminder to myself to fix.