Deploy into `dropins`, not `plugins`.

I have tried installing/deploying eclim using only ant, without having
used the installer before.

Eclipse did not detect the plugins, until I moved them manually from
plugins/ to dropins/.

I think the ant task (or a new, documented one) should therefore deploy
to dropins instead.

I think it would be better to rename the property / use a different one for the deploy target, instead of letting "plugins" point at "dropins".

This was reported initially as issue #44, with pull request #45.
Both are quite flooded with administrative comments, therefore I create
a new pull request, using a separate branch.