Make the URI constructor default

After much wrangling with submodules, I've finally been able to push changes in my local repo to my GitHub fork.

I've also tested this with the latest libc++ and can confirm that the libc++ that comes with Xcode 4.5 is broken with `std::error_category` not having a default constructor. This change essentially makes it so that whatever the base default constructor is, the derived class will just invoke it.