Markers: AttributeError when calling __repr__ / __str__

[elabrosseRodeofx]

The otio.schema.Marker object's __repr__ and __str__ methods are referencing a 'media_reference' and a 'source_range' attribute.
however Markers do not have those attributes. They do however have a 'marked_range' attribute that is not referenced in the __repr__ / __str__

Printing a Marker instance results in an AttributeError:

>>> otio.schema.Marker()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/devLocal/elabrosse/rdoenv/auto_lineup/otio/lib/python2.7/site-packages/opentimelineio/schema/marker.py", line 25, in __repr__
    repr(self.media_reference),
AttributeError: 'opentimelineio._otio.Marker' object has no attribute 'media_reference'

From what I see it seems that the content of opentimelineio/schema/clip.py was copy/pasted almost as is in opentimelineio/schema/marker.py.

My corporation is waiting for confirmation that our CLA was received. We will submit a PR as soon as possible.

[jminor]

Yep, we got the CLA. Thanks for finding this oversight.

[elabrosseRodeofx]

Made a PR:

#596

[reinecke]

I accidentally resolved this in #740 after it'd already been resolved in #596 (Oops!)