Re-enable signing and tycho-p2-plugin, switch website to antrun

See bug 551450. The wrong zip file ends up being used as the build
artifact for the documentation plugin when using maven-assembly-plugin.
Using maven-antrun plugin instead allows us to workaround the issue
and have EMF Compare's doc plugin be signed correctly.

Change-Id: Ib6656920ea2b5fea6be03b338d61a8ad0abebbca
5 files changed