Changed context root, and made changing this easier.

Context root is now /opensonar_whitelab (the name we use on the
production server).

defines.js now defines two symbols: one with the context root and one
with the full base URL. defines.js gets included by header.vm, and URLs
are constructed in Javascript using these symbols only.

build.xml, the Eclipse project and the .properties files were updated.
The way WhiteLab.java dispatches requests was fixed (extraction pathname
doesn't always equal context root, e.g. when testing from Eclipse).