Fixes #153 - Scalac plugins not fed to eclipse.

This fixes the issues as described:

* Xplugin/P settings can be specified multiple times
* Pull scalacOptions from "compile" configuration, most likely the ones we want for eclipse
* Eclipse cannot support specifying the continuations plugin currently, because it
automagically includes its own.
Adding reference scala issue so we know when we can remove the hack.
Merge pull request #158 from typesafehub/fix/scalac-plugins

Fixes #153 - Scalac plugins not fed to eclipse.