Added URL params (request.url_params)

URL params are now present as a `ci_map` member variable of `request`.
* Supports `&` and `;` delimiters in any combination (`?foo=bar&hello=world;black=white`)
* Supports assignments (`?foo=bar`) and single keys (`?foobar`)
@ipkn I have looked at a handful of libraries including uriparser, which is pure C and so I don't think really fits in with crow's style, [httpqueryparser](http://sourceforge.net/projects/httpqueryparser/) and [cpp-netlib](http://cpp-netlib.org/). The latter two don't even break down URI query strings, they just isolate them in the URI.

I did come across this one - [qs_parse](https://github.com/bartgrantham/qs_parse) - which is also C, but small enough to be wrapped. Will that do?