Changes to URI

Dean,

I made some breaking changes to the URI, but none of these changes should affect cpp-netlib adversely.

Most of the changes are a result of ongoing discussions I've had with the BSI C++ panel, attendees at ACCU at Bristol and the feedback I've had so far from the ISO committee.

The changes I've made ought to be clear from the commit messages, but please ask if anything isn't up to standard.