Update requirements section of docs
Update requirements section of docs

* Rephrase summary of hard requirements and listed version numbers
* Remove excess verbiage describing some version number issues, deferring
  to the summary instead
* Move matplotlib from required to recommended
* Mention Python 3 compatibility
* Update minimum BioNetGen version
* Add minimum Kappa version
* Add instructions for installing BioNetGen and Kappa for PySB use
* Update some URLs

johnbachman/pysb,LoLab-VU/pysb,johnbachman/pysb,LoLab-VU/pysb,pysb/pysb,alubbock/pysb,jmuhlich/pysb