Use setImmediate instead of process.nextTick.

This depends on caolan/nodeunit#195 to fix the errors with recursive process.nodeTick calls as of [Node v0.10.0](http://blog.nodejs.org/2013/03/11/node-v0-10-0-stable/).

Also depends on brianmcd/contextify#61 to get a working version of contextify built on 0.10.0.

But, with both of those linked to node_modules, 100% success on the tests with this change.