Remove rl dependency.

It's unmaintained, which makes it a liability for us for just one
file.  It has served us well, but is `private[util]` so we don't make
any promise to support it oursellves.
Added property that urlDecode(urlEncode(s)) == s.
Update unnecessarily scary comment about 4-byte characters.
Merge pull request #99 from http4s/topic/remove-rl

Remove rl dependency.