Highlights:

• Process nested arrays recursively
• Convert other values by casting to (string) (this takes care of Horde_Url)
• Do not output consecutive numeric indexes in arrays
• Do not skip parameters with empty values
• Do not encode [ and ]

This also fixes the test failures mentioned in #2.

With this PR, instances of classes without __toString() will not be encoded the way http_build_query would encode them, but this is how it was before 523d5e5, anyway.

More tests should be added, specifically to test nested arrays (possible with Horde_Url), e.g.

    $url->add('x', [ 'abc', 'def', [ 'k1' => 'v1', 'k2' => 'v2 '] ];