Added linter and compiler warnings to build.sbt

Ref #79.
I've commented out value-discard, as it has 5 false positives.

You might want to take a look at https://github.com/HairyFotr/linter/blob/master/staticanalysis.sbt
Wartremover was broken when I was writing this, and cpd/findbugs aren't usually very relevant to scala (probably not enough to be in the main build.sbt).