Add WeakMap.has and conform WeakMap.delete to spec

This builds on top of the test suite added in pull request #5 to:

- add a WeakMap.has method, which indicates the presence of a key/value combination [0]
- alter WeakMap.delete to return true/false, indicating if a key/value combination was removed [1]

References:
[0] - http://people.mozilla.org/~jorendorff/es6-draft.html#sec-weakmap.prototype.has
[1] - http://people.mozilla.org/~jorendorff/es6-draft.html#sec-weakmap.prototype.delete