Deprecate ScpURI.parse/.scp?; Speed up URI.parse.

The URI.parse method was parsing the URI 2-3 times depending on
the format of the URI received. By combining the parsing for scp
URIs and normal URIs, we can optimize the way each of them is
handled, speeding up both.
Use the rspec expect syntax
Use more of Addressable's parser for a bit more speedup