Minor Cleanup/Additions to ElapsedTime work

* Added support to the tomcat server adaptor for correctly getting request start time. (Includes additional dependency on tomcat-coyote)
* Update layouts.html to document the addition of elapsedTime/D to the pattern support for IAccessEvent
* Modified AccessEvent.java to calculate the elapsedTime at construction of the event, instead of repeatedly in the getter.  This allows the value to be more easily seen in a debugger, and cleans up the code slightly.