Added the Nexus driver

1. This is not tested, because there is no Nexus switch in the haas environment 
2. The Nexus driver possibly works only with one switch. This would be the case if the Nexus switch does not support switch stacking.
I'd just like to note that pull request #230 fixes a bug here for the Dell driver and is pending review.  It seems logical to me that #230 should be merged first, then this can be fixed and merged. @gsilvis @zenhack?