Logging improvements

* You can now (finally!) override the log source and lineno.
  * Implemented in ``fixtures/log.py``, inspired by @psav's #433
* Explained the relationship between logger names and logger confs,
  as discussed when the perflog was introduced.
add separator for logging test start

Conflicts:
	fixtures/log.py
Give me a line of equals signs, Vasili. One line only, please.
Log marker updates

* Added test name into marker
* Added format_marker function to aid making markers
* Added Start of test run marker
Merge pull request #438 from psav/logging

Logging - more enhancements?