Allow INI/JSON/YAML configuration for logging

Add a new option --logger-config option to circusd and a matching loggerconfig
option to the circus.ini file.  These options can be pointed to a file
which is read by the standard Python logging.config.fromFile (INI file)
or logging.config.dictConfig (JSON or YAML file) functions.

This allows an external file to be used to configure the logging in circusd
and exposes the full power of the built-in Python logging framework.
For example, custom log handlers and formatters can be specified and output
can be sent to syslog if desired.  The use of an external file means that
the logging configuration can be shared by multiple frameworks.  For example,
the same YAML file could be used for curcusd and also read by a Django
settings.py file.

Documentation: Minor updates as necessary

Testing: No existing tests modified.  New tests added which invoke circusd
as a child process with the various logging options configured.  Output
to the configured log file is verified.