Change how the signature base string is encoded per OAuth standards

Previous method of encoding (adapted from AFHTTPClient's AFPercentEscapedQueryStringPairMemberFromStringWithEncoding) is not suitable for constructing a consistent base string to generate the signature from.

In particular, passwords with special symbols such as asterisks, and I believe square brackets and tilde will fail the signature match.

The final OAuth spec on percent encoding (http://tools.ietf.org/html/rfc5849#section-3.6) specifies that all unreserved character must not be encoded (per RFC 3986), while all other symbols must be encoded (which differs from RFC 3986).

In my tests following RFC 3986 by not encoding the unreserved characters seems to work fine.