Cross building support

I added the ability to cross build the SBT eclipse plugin, so it can now
be built for SBT 0.13 easily.  Yes, there is a cross building plugin for
SBT, however this needs to have knowledge of all versions of SBT that
you want to build against.  It knows nothing about SBT 0.13.0-M2 yet,
for example.  So it's no good for building against milestone versions of
SBT.