Give mounted app access to its mount path (#4).

[clintwood]

I need this for mounted site... unless there is another way to resolve URLs?

[tj]

#1 would be probably the best for logging etc and this case

[clintwood]

@visionmedia, if by #1 you mean full url, koajs already has this.originalUrl and what is difficult to infer from app.use(mount('/prefix', webapp)); inside webapp is /prefix.

So for logging etc. we just use this.originalUrl but we still need the mount prefix.

I've renamed .mountPath to .mountPrefix to be more clear on intent.

[tj]

oops forgot we added it in core. I like .mountPath better actually since we don't just check the prefix anymore, we used to. I think we should just leave the path passed as-is though no stripping of trailing slash

[clintwood]

Done... Not sure what happened there... squashed previous commit and now I have this...

[tj]

weird! added it and released