http instrumentation: capture URL at start.

This ensures that badly-behaved frameworks like Connect that munge the
request URL don't rename metrics before the instrumentation has a chance
to capture it.

Closes newrelic/node-newrelic#35, and this version has tests.
Updated documentation for new version.
Released 0.9.22
Merge pull request #2 from newrelic/master

0.9.22