mozlog: move the capture io class from web-platform/tests to mozlog (bug 1021926) r=jgraham

The ability to capture the parent process' stdio is suggested to be a useful feature
to move from web-platform/tests into mozlog. To do so, I have created a new capture.py
file within mozlog/mozlog. This includes the CaptureIO class and its dependencies,
including the LoggingWrapper and LogThread classes. These have been removed from their
original location, to avoid duplication, and the files depending on them updated
accordingly.

It would be useful to add unittests testing the CaptureIO enter and exit methods, and
the original_stdio, logging_queue and logging_thread properties. I have begun such a
file with test_capture.py in mozlog/tests. This is a work in progress, however I may
need some guidance, please, in regards to creating appropriate mock data to assert.

Differential Revision: https://phabricator.services.mozilla.com/D22166