Make Thespian's internal logging threshold configurable

[danielmitterdorfer]

With this commit, Thespian recognizes a new environment variable THESPLOG_THRESHOLD to configure the minimum log level which is written to Thespian's internal log file. The default value, logging.WARNING, stays as is.

We also log deserialization error on ERROR level.

Closes #23

[danielmitterdorfer]

A note on _name_to_level in utilitis.py: Although the logging module contains a similar dictionary I did not use it as it is considered an implementation detail and not part of the official API.

[danielmitterdorfer]

I had a look at the build failure in https://travis-ci.org/kquick/Thespian/builds/379058936. The Python 3.6 build failed with:

in test_threadedActor.py

_____________ ERROR at setup of test_threads[multiprocTCPBase-100] _____________
E           thespian.actors.InvalidActorAddress: ActorAddr-(T|:24252) is not a valid ActorSystem admin

It looks unrelated to my PR to me.

[kquick]

Yes, that's unrelated. Occasionally there are port collisions between the ports chosen by the test code and something that is already using the port.

[kquick]

This looks great. Thanks, Daniel. I'll get this incorporated into a release by the end of next week. -Kevin

[danielmitterdorfer]

Thanks Kevin!