Allow users to supply a logging formatter

In order to ease aggregation and analysis of logging information a
consistent logging format is necessary, this commit allows users to pass
in a logging formatter of their choice to unify Qless' logging output
with the output of their application.
Remove order dependency in the serial worker spec

Signed-off-by: Tyler Pickett <t.pickett66@gmail.com>
Merge pull request #205 from tpickett66/logging-changes

Allow users to supply a logging formatter