Update Client URLs to support a ROOT_URL subdirectory, if set

Hello,

I had an requirement to allow for multiple meteor app instances on one domain name, eg

    domain.com/app1/
    and
    domain.com/app2/ 

etc.

And also I wanted to use collectionFS on those apps.

Now I saw that the URL endpoints didn't really respect the /appX/ - subdirectory if a ROOT_URL with a subdirectory is supplied to the meteor instance.

So I patched this in for the client side URL generation and with this it seems to be working for me.

I'll post a pull request to Iron Router regarding this too, if you are interested in using this as well.

Note: I only tested this with the File Storage Backend and HTTP Up / Download.

Thanks for your continued support (and the emergency tutorial for making the jump / surviving the fall to 0.9 too :) )