DOC: beef up documentation (attributes and examples section)

[scott-huberty]

closes #23

• Adds an Attributes section to the eyelinkio.edf.EDF class, and details the info and discrete attributes`, including all the keys within each of them that are populated by the reader.
• adds multiple Examples sections to the eyelinkio.edf.EDF class, including for theto_mne and to_pandas methods. This means that with the current pytest configuration, pytest will test these docstring examples when you invoke the pytest command.

[scott-huberty]

A little annoying that the EDF file does not store the time zone of the computer it records from.

In our Eyelink ASCII reader in MNE, we forcefully set the date time as UTC, even though we know that the datetime is actually in some local time-zone. I think we did that just to be compliant with MNE's info["meas_date"] standard (must be in UTC).

But here, we just keep the datetime timezone-naive, and I've left a little note about that.

@teonbrooks I'm wondering if you have an opinion on which approach you'd prefer for your data?