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.