Mark generator functions with wrapGenerator.mark.

This hook will likely prove useful in more ways in the future, but for now
it's primarily useful for setting the .constructor property of the
generator function to the GeneratorFunction constructor in order to
support wrapGenerator.isGeneratorFunction.

Closes #48.
Regenerate runtime/min.js to incorporate wrapGenerator.mark changes.
Prepend wrapGenerator.mark calls to closest enclosing BlockStatement.
Merge pull request #55 from facebook/GeneratorFunction-constructor

Support wrapGenerator.isGeneratorFunction.