Add createdAt accessor to DataSet about pbcore

[mpkocher]

Fyi, Herb should be adding iso8601 python package to the build which can be used to parse the created at field. This should simplify the conversion to datetime obj.

http://bugzilla.nanofluidics.com/show_bug.cgi?id=30186

[dalexander]

Not interested in adding additional dependencies to pbcore for minor features, if that is in the cards here... but I am open to reasonable debate on that

[mdsmith]

Resolved by pull request #45