Re-enable compileTimeOnly and still support 2.10

Added a source generator in sbt that will generate it for 2.10 tasks.
Downside is that this might conflict if other people in 2.10 land are using this trick.

TODO: Figure out how to add unit tests that will pass when the compile fails (and then only expect it to fail in 2.11)
I'd add an `"only"` in there just to make it more clear