(PE-2329) add a simple plugin mechanism to TK

Plugins are specified as a directory containing a set of .jars using the
--plugins CLI arg.  The plugin .jars are scanned and checked for
duplicate classes/namespaces and, if none are found, the plugin .jars
are added to the classpath.
(PE-2329) exclude servlet API from jetty

It is already in the dep tree elsewhere, and the duplicate detector
found duplicate classes on the classpath.