Adding more robust query string parsing.

This commit adds a QueryParser class that is able to perform much more
robust query parsing. The query parser can track whether or not to use
a duplicate aggregator and whether or not to use numeric indices in
mutli-value key value pairs. You can now control how the query string
is url-decoded when parsing as well as disable url-decoding.

This change addresses nested key value pairs using 'a[b][c]=d' syntax
such that when parsed, will produce nested arrays containing
`['a' => ['b' => ['c' => 'd']]]`. This gets Guzzle query strings more
like PHP's parse_str().

This change closes #730 and closes #731 dealing with multipart POST
requests stripping "+" signs from values. POST key value pairs for
multipart requests are no longer modified at all, but rather sent over
the wire exactly as given.