This adds the option to enable `pushState` simulation support in Express

I figured it would be nice for Lineman to be able to simulate what configuring a server with HTML5 PushState _and_ running a single page app would look like. This was surprisingly easy :+1: 

Since express doesn't handle 404 or any other errors explicitly this simply exploits that fact and adds 1 final piece of middleware to the chain; this has the effect of serving up `generated/index.html` on any request that would yield a 404

In conjunction with enabling pushState for Backbone/Angular/Embers routing abstractions this basically simulates what having a server configured to work with HTML5 pushState works like. Yay? :)

This doesn't modify any behaviour unless the user adds a simple line to `config/application` as follows:

```javascript
server: {
    pushState: true
}
```

I tested this with a couple of apps and it seems to work pretty well but I wanted to discuss before merging. Seems pretty low risk to me given no effects take place unless users explicitly add this in the config.

Thoughts from @searls, @kbaribeau, @jasonkarns welcome :)