Helpful error when missing cemerick.cljs.test

Display a helpful error message instead of
"ReferenceError: Can't find variable: cemerick"
when either there were no test files included in
the build or when they did not require
cemerick.cljs.test.

Based on #51 and the latest code, improved to also kick in if there is no test file and to warn that
Node does not work with none/whitespace optimizations.