Support for elapsed time to jetty and tomcat access logs

I was working with @francoisforster to include tomcat support, plus some minor enhancements to his jetty work.  However, he has stopped responding to any correspondence, and we are in need of these features very soon.  We are unable to wait any more while he has been non-communicative.  

This pull request includes all of @francoisforster's work (https://github.com/qos-ch/logback/pull/145), as well as the changes I made to support tomcat (https://github.com/francoisforster/logback/pull/1).

However, since he was not communicating with me, I was afraid, he might not be communicating with the logback maintainers as well.  

If there is a question about where the capital D for the elapsed time came from, you can look at the tomcat access log documentation [here](http://tomcat.apache.org/tomcat-7.0-doc/config/valve.html#Access_Log_Valve/Attributes) or in the [apache docs](http://httpd.apache.org/docs/2.2/mod/mod_log_config.html#formats)

I am only making this pull request as an attempt to get communications moving again.  If @francoisforster is actively communicating with the logback maintainers, then I have no intention of confusing that issue, but if you need any questions answered, I would be happy to discuss or work through any issues.  

Thanks.