Rename Parser.parse() to better reflect that it now returns the labels

[RyanBalfanz]

In response to closed issue #5 (http://github.com/RyanBalfanz/PyPDS/issues/closed#issue/5), Parser.parse() might be better named to indicate that it returns the labels.

• .parse_labels

Or, possibly the label property should be resurrected to return parse() (or whatever the new name may be), thus supporting both methods.