Implement listing of tagged VLANs on a given Port
add methods get_ports and get_vlans for switch
add description property to port

* ifDescr contains the name of the port used by the switch
  (e. g. A11, 2 or TrK1 for Trunks)
Merge pull request #1 from f1ori/master

add some convenience methods
Tweak docstrings a bit for consistency