Add options flag to include empty values

Currently, when parsing a querystring, empty values are not included in
the result. This adds a flag, `includeEmptyValues`, to the (new)
`options` object (express/connect already sends its `options` object
here) allowing for the result to include empty values.

2 new tests added.
README.md updated to include an example.
./querystring.js has been rebuilt and all `./test/browser` scripts and
styles have also been updated.