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
fix(testing): create_environ does not decode the path

WSGI servers decode the path submitted in the request before passing it
to the app via the PATH_INFO environ field (wsgiref, uWSGI, and gunicorn
were tested to verify this behavior). This patch updates create_environ
to behave similarly.
Merge pull request #339 from kgriffs/issues/261

Issues/261
Merge pull request #341 from kgriffs/environ

fix(testing): create_environ does not decode the path
chore(HTTPError): Clean up 405 and DRY has_representation model

This patch modifies HTTPMethodNotAllowed so that it no longer supports
adding details (since it is not common/idiomatic to return a body with
HTTP 405 responses).

BREAKING CHANGE:
    HTTPMethodNotAllowed no longer supports setting a title or
    any other HTTPError kwargs.
doc(falcon.exceptions): Clean up docs

This patch implements the following:

    * Use "e.g." instead of "for example"
    * Document class names in the reference docs under falcon instead
      of falcon.exceptions
    * Add the NoRepresentation mixin to the reference docs
feat(falcon.exceptions): Improve HTTPInvalid* and HTTPMissing*

This patch cleans up HTTPInvalidHeader and HTTPInvalidParam, and adds
two additional, distinct classes to use when the header or param is
missing altogether.