Bug 946083 - Part 1: Delete .class files when (re-)building a Java JAR. r=glandium, a=sylvestre

This cleans up stale .class files, so they don't get packaged into the
.jar files that Proguard consumes.

--HG--
extra : source : 5c68bbcef1abeec86d0b2e51ec842f51907a9155
extra : amend_source : f1addb76902102db9f23d650e6a6df90ddaed55b
Bug 946083 - Part 2: Add dependencies for Proguard input jars. r=glandium, a=sylvestre

This restores no-op builds.

--HG--
extra : source : 1a05d8dffc6541b985d4495bacc41d87303031f9
extra : amend_source : f32018f91a79154547a0840501d7bbaac22688d0