perf(Request): Don't normalize CONTENT_* headers

Rather than always normalizing these headers up front, this patch
simply uses the names as they already exist in env. It was found
that this makes Request.__init__ take slightly less time to execute.

Issue #261
perf(Request): Misc. minor performance enhancements

This patch contains a variety of minor performance enhancements for
the Request class. In the course of making these changes, behavior
of empty strings given for headers was changed to be more strict.

BREAKING CHANGE:
    If a header is given in the WSGI environment, but has an empty
    value, the falcon.Request convenience method for accessing that
    header will not raise InvalidHeader instead of simply returning
    None, since this condition is exceptional.

Issue #261
perf(uri): Speed up uri.parse_query_string

This patch shaves some microseconds off uri.parse_query_string by
switching to using a combination of str.split and str.partition.

Closes #261
Merge pull request #339 from kgriffs/issues/261

Issues/261