Fix node 0.11 incompatibility in w3c test harness.

The rewritten `vm` package in node 0.11 doesn't create a new unique
context for every call to vm.createContext(foo) any more; instead it
just "contextifies" foo. This caused confusion as every test suite was
sharing the same context.

Do a shallow clone into the context to fix the issue.
Update mocha/should dependencies.

Tested that latest releases of `mocha` and `should` do not cause any
problems with domino's test suite.
Merge pull request #40 from cscott/update-deps

Update mocha/should dependencies; fix node 0.11 incompatibility.