Adding support for "hard" inplace mode 

Hello again!

Here's another feature proposal with some reference implementation. :)

When developing an application with very large (>100 MB) webapp (src/main/webapp) folder one should have a choice whether to copy this folder into temporary folder (build/inplaceWebapp) (as Gretty does) and have support of overlays and resource filtering or serve webapp folder directly from source directory (as maven-jetty-plugin does). 

Direct usage of src/main/webapp ("hard inplaceMode" as I called it) have following benefits:
* Zero latency for user to see changes in webapp folder. 
* Lower startup time because Gretty doesn't need to copy whole src/main/webapp folder into temporary directory.
* Lower application restart time on full/fast reload (same reason).

Of course, hard inplace mode has some disadvantages:
* Unable to use overlays and filtering for obvious reasons.

P.S. Key and allowed values of inplaceMode property is a subject of discussion. :)
P.P.S. Another thing I want to discuss is should Gretty notify user that some features won't take place in hard inplaceMode or it's only thing for documentation.