Couple fixes for 0.1.x series

Hi @blakeembrey these are the changes that, if released as like a 0.1.x series, would be awesome to have for express 4.x series (specifically, for https://github.com/visionmedia/express/issues/2207).

The two commits in here by you are just e3df26313d4ee1ddb3fec9389ad0f1c43598b980 split in half (I thought I only needed one half, but apparently did need both parts of that commit).

I also made an additional commit that is also needed to use it with `app.use`.

You had also brought up an issue with `mergeParams` option in https://github.com/visionmedia/express/issues/2207#issuecomment-48078903 so if they relate to this, let me know :) These changes I have tested against express 4.x with a patch to support express issue 2207 and they all pass.

As for the numeric index issue, the numberic indicies I was supporting was from the line https://github.com/visionmedia/express/blob/master/lib/router/layer.js#L53 , which is always in order without holes.