Added functionality to close proxy.

[unilaterus]

Added function to wrap closing the internal server. This function accepts an optional callback. The close will immediately cause proxy to stop listening on its current port but it will not close any pre-existing connections. Once all open connections are closed it will execute the given callback if one was supplied.

Closes issue: #675

[jcrugzz]

@unilaterus Thanks for the contribution! This looks good, the only thing I see potentially dangerous here is us nulling out the server object before the callback is executed (if one was passed in). I believe there should be a server.once('close') event that can be listened to to null the server after its fully closed or we could just wrap the callback and null the server before responding to the original callback that is passed in.

[unilaterus]

@jcrugzz Nice catch there. I hadn't thought of that. I pushed up a revision to correct that issue. I chose to go with the wrap the callback option because I'm not entirely sure if listening for close event will work since I think the callback passed in replaces the standard close event.

[jcrugzz]

@unilaterus thanks!