Feature/endpoint stack

[sfsekaran]

One thing I've noticed is that some specs don't pass anymore (some issue with ports). I haven't tried to ameliorate that issue.

However, I have added a few passing specs and code which add some useful functionality. Namely, making it so @Artifice.activate_with(q) do ... end@ restores the previous rack endpoint after the end of the block if Artifice was already activated when the @activate_with@ block was called.

We needed our Artifice blocks not to deactivate artifice after running because we were using a global endpoint and specific endpoints in our tests.

I've updated the README so you can see what the use cases are.