Add motif discovery tool discrover to overlay

Discrover is a tool that uses discriminative learning to discover sequence motif of protein binding sites in nucleic acid sequences. The [paper](http://nar.oxfordjournals.org/content/42/21/12995) (open access) describing the method has just been published in the journal Nucleic Acid Research.
I'm the author of both the paper and the tool, and would love to see ebuilds for Discrover included in this overlay. Should there be are any issues with my pull request, please let me know so they can be resolved quickly!

At this point I've got two questions:
1. regarding the metadata.xml file. I've not added a herd tag, but state myself as maintainer of these ebuilds. Is that how it's supposed to be?
2. considering future releases of Discrover: should I add them to my fork of the overlay and send new pull requests for them? Also: what's a customary number of previous versions to keep around for a package?
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