ES6 Yield expression

This adds support for `yield` *if* "harmony" branch of esprima is used. 

Since harmony version of esprima is unreleased yet I haven't added it as a dependency. This change is future-proofing at this point (but if you [upgrade esprima](https://github.com/pornel/istanbul/commit/7a12a249ba49a083e1e97d4cf26aa8b22cde07bd) it works).