Added support to handle empty query parameters.

Added support to handle empty query parameters through a global
option set ``RequestOptions`` applied to ``API``.
This option can be enabled by using
api.req_options.keep_blank_qs_values = True where api is an instance
of ``API``.

This change allows queries such as '?a=1&bool' and '?a=1&empty=' to
preserve the 'bool' and 'empty' values respectively as '' instead
of the default value of None.

Another option was added to ``Request.get_param_as_bool`` to allow
empty query parameters to return a True value.
Because of the non-obvious nature of this change ('' == True), it
requires a blank_as_true=True argument be passed to
``Request.get_param_as_bool``.
Merge pull request #336 from xiaclo/patch-2

Store blank parameters as '' instead of None
fix(Request): Improve support for content-length under wsgiref

Under wsgiref, if no content-length header is provided, an empty
string is inserted. Update falcon.Request to handle this case,
as well as to more gracefully handle the case that the header is
set to any invalid value.
refactor(Request): Fix indentation of _parse_form_urlencoded

Remove the extra indentation level of the _parse_form_urlencoded
method's body.