Addressible's URI parsing shows up at the top of the profiler for our use case.

This patch adds a cache to JSON::Util::URI.normalized_uri, to trade CPU time for memory.

There are other expensive Addressible methods that should be cached elsewhere.

benchmark: https://github.com/mjc/json-schema-perf

MRI 2.2.2 before: 825.363  (± 6.2%) i/s
MRI 2.2.2 after:  1.436k (± 4.5%) i/s

1.74x faster

JRuby 1.7.20 before: 1.445k (± 2.5%) i/s
JRuby 1.7.20 after: 2.272k (± 6.1%) i/s

1.57x faster

Addressable::URI#fragment= triggers a revalidation of the URL,
which is super slow.  Don't do this when it isn't necessary.

This reverts commit ac24f48.