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

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

--HG--
extra : rebase_source : a090d707579711f85b8f9f174b88bf9302be0b24
Bug 946083 - Part 2: Add dependencies for Proguard input jars. r=glandium

This restores no-op builds.

--HG--
extra : rebase_source : 0a3554f9d7070c3448eaf9536e18adab6b5781d6