[1.9] Add "Mark" features to Psych

Not sure when this will come along, but Aaron Patterson is adding some enhancements to his Psych parser. Among them are:

• a Mark class, similar to SnakeYAML's
• Access to all that Mark exposes, like line, column, and index

SnakeYAML will need an update, since it currently does not expose "offset" (as "index") in its Mark class: http://code.google.com/p/snakeyaml/issues/detail?id=104

A quick patch to add line and column to Parser is here: https://gist.github.com/e160ed9a5e909da6f821

Note that if a Mark class is added, the above patch will be unnecessary (since we'll add a Mark class as well).