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