perf(Request): Optimize instantiation

This patch reduces the time it takes to instantiate a Request object
by a few microseconds by implementing lazy header parsing from the
WSGI environ.

This should not impact reading headers significantly, so this change
should be a net win.

Kudos to the OpenStack Swift team. This idea came from my research into
their swob module.

Closes #213
Merge pull request #217 from kgriffs/lazy_header_parsing

perf(Request): Optimize instantiation