geni_trust consistency fixes

Corrected mysterious geni_trust plugin inconsistencies. Also added simple presence check for xmlsec1 functionality.

Includes requirements.txt file for use with `pip`. However, I could not update the documentation in the wiki. I have updated my forked wiki, but it looks like I can't use a pull request to include the changes: [https://github.com/gollum/gollum/issues/265](https://github.com/gollum/gollum/issues/265)