Fix a bunch of issues with Mininet adapeter (closes #12)

This commit fixes the following issues with the Mininet adapter.

 * Rename Mininet example from mininet.py to mininet_datacenter.py
   because it would not run as a result of conflict with mininet
   namespace
 * In Mininet example use an example topology with no loops because
   Mininet's default controllers do not support topologies with
   loops or multiple paths.
 * In Mininet example correct the names of relabeled nodes
 * Fix relabeling of nodes in to_mininet function to comply with
   Mininet naming conventions.
 * Clarify in documentation and code Mininet's issues with loops
   and multipath topologies.