New annotation @venus-code

* `@venus-code` should be used to include the source code file that is under test.
* Files included with `@venus-code` will be instrumented for code coverage when `--coverage` flag is set
* Files included with `@venus-include` will never be instrumented for coverage
Merge pull request #258 from sethmcl/master

New annotation for code coverage `@venus-code`