Added callback feature that fires before the first tree registration
occurs.  This let's the calling code with the tree and the config so
that it can then register logging for anything internal that does a
large amount of non-trivial processing.

We were getting errors that were buried in the nject resolution and so
needed to add logging to nject, but the logging needed to flow out to
any log file which is configured with the configuration file
express-train provides by default.
Merge pull request #40 from lcaballero/add-logging-callbacks

Added callback feature that fires before the first tree registration