fix(Request): Improve parsing of application/x-www-form-urlencoded

This patch adds error handling for non-ASCII form request bodies. If
an improperly-encoded request is submitted, Falcon will log a message
to the WSGI error log and ignore the request body.

The docstring for Request was updated to provide more information
regarding Falcon's parsing of application/x-www-form-urlencoded,
especially regarding the client's responsibility to properly
encode the request body.

Closes #313
fix(Request): Be more lenient regarding query string param names

This patch allows a wider variety of characters to be used in
query string params, while also slightly improving the performance
of parsing the same.
Merge pull request #316 from kgriffs/issues/313

fix(Request): Improve parsing of application/x-www-form-urlencoded
Merge pull request #317 from kgriffs/qs_parsing

fix(Request): Be more lenient regarding query string param names