Adding the QueryItem struct in path

[yageek]

As requested in #206, building an URI from a builder should have a smaller granularity.
A struct QueryItem has been added. It holds the elements of a query component.
A method new inside PathAndQuery has been added. It that requires a path and a list of QueryItem and build a new PathAndQuery from them.

The URI builder could hold both a String and a Vec<QueryItem> fields instead of one parts field and could provide new methods to configure the path and the query separately.

[seanmonstar]

Thanks @yageek for thinking about this! I think we should discuss a bit more what the API should look like.

• Assuming that this should be an addition to the Builder, we should consider what new method(s) should be added.
• I'd suggest that this shouldn't (yet) add PathAndQuery::new, as that's a pretty coveted constructor name. We should start with just the Builder.
• It's preferred that any public struct have its fields be private, so that internal representations can be changed without worry of breaking changes.

(If you'd rather discuss these on the tracking issue #206, that's fine too!)

[yageek]

I think you're right, I may attack the problem in the wrong order. I'll put some comments on #206