Switch to IDEA's Scala plugin

I've implemented a switch to IDEA's official Scala [plugin](http://plugins.jetbrains.com/plugin/1347) over the IDEA plugin for SBT, as this should be a more future proof solution (given support from IDEA's developers). I've tested working on Adept with this plugin, and it appears to work well. For instance, I can easily build Adept from IDEA. IDEA is also supposed to automatically keep the project in sync if SBT configuration should change.