Follow HTML 5 draft and avoid RFC 2231 by default.

This passes the request object along for field rendering. Two attributes of the request object control that rendering:

* `field_encoding_style` which may be `HTML5` or `RFC2231` controls
  the encoding of non-ascii field names and file names.
* `form_data_encoding` can be set to encode the form with something other than UTF-8.

See [the current (as of this commit) draft version of the HTML 5 standard](http://www.w3.org/TR/2013/WD-html51-20130528/) for details on their idea of [how to format multipart/form-data](http://www.w3.org/TR/2013/WD-html51-20130528/forms.html#multipart-form-data).

These changes addresses shazow/urllib3#303.

[1]: http://www.w3.org/TR/2013/WD-html51-20130528/forms.html#multipart-form-data