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
i was more interested in feedback for the rest of the patch, does that look ok? ideally, dbuild would have a property for `ivy-home` (see https://github.com/scala-ide/uber-build/blob/master/zinc/bin/dbuild.properties), then we would not need to hijack `user.home`.

`dbuild.boot.directory` and `dbuild.cache.dir` are ok, i guess.