Detect and ignore rpdb invocations on same port during rpdb session.

* An exception is still raised if the port is occupied by something other
  than rpdb.
* We also provide for multiple simultaneous rpdb sessions on different ports.
* HOWEVER, the detection data is not yet handled in a thread-safe way.
Include thread-safety caveat.
Make recurrent rpdb session handling thread-safe.
Note easy option to manually install, and clarify Esc+q to escape telnet.
Register changes in CHANGES.txt and increment to 0.1.6

(The changes include the other pull request.)
Use handle's id instead of handle in OccupiedPorts, stay back at 0.1.5.

Use the handle's id to avoid unnecessarily storing a reference to the
handle object beyond its lifetime.

Also, adhere more closely to PEP-8
Merge pull request #10 from kenmanheimer/master

Make repeated rpdb.set_trace() on the same port while within rpdb.set_trace() a no-op