Initial implementation of usage logging

Earlier commits are also in #475.  This changeset is based on work in trello-44-logging that has not yet been merged.

This changeset adds a class to record usage events (EventLogger).  It logs to the dryad-usage.log file.
Each entry includes date/time, session ID, eperson email (if logged in), action (e.g. page/step) and additional context information supplied by the caller.

A few areas of Dryad have been targeted to add logging - search, submission, and the login page.  More will be added, this is merely a first draft.