Add tool citations.

New feature included in Galaxy with commit https://bitbucket.org/galaxy/galaxy-central/commits/8536414e03580828831dbd68ba6fd8a888630cd9. This will be included in the Galaxy's August 2014 release. For more information see Galaxy Pull Request #440 (https://bitbucket.org/galaxy/galaxy-central/pull-request/440/initial-bibtex-doi-citation-support-in/diff).

Because Galaxy's XML tool parsing is very relaxed, these extra tags will not break older Galaxy instances but such instances will not render these citations.
Clarify citation TODO notes
Clarify status of this experimental tool (now superceded)
Acknowledge @nsoranzo in README file for masking datatypes