added new data sources

* [usgovxml.com](http://usgovxml.com/)
* [enigma.io](http://enigma.io/)
* [datahub.io](http://datahub.io/)
* [aws.amazon.com/datasets](http://aws.amazon.com/datasets)
* [databib.org](http://databib.org/)
* [datacite.org](http://www.datacite.org)
* [quandl.com](https://www.quandl.com/)