local .dbuild directory

cc @huitseeker 

dbuild uses the `~/.dbuild/[boot/cache-0.9.2/ivy]` directories, which is problematic when running multiple builds on the same user account (according to @cunei). Something like the hack here should help, again according to Toni
Actually: does dbuild even know about these properties? Or is it enough if I introduce a new property here, and just use it in https://github.com/scala-ide/uber-build/blob/master/zinc/bin/dbuild.properties ?