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.
add note re: file and expression evaluation is optional, fixes gh-57
0.3.1
0.3.2-SNAPSHOT
need to refer to current version in self-plugin profile
Adds support for "--test-data" (file reading via the test commands)