This commit changes the routing algorithm. In particular, it enforces
precise matching of formats. With this change, a route with a specified
format only matches requests that have the same format specified. A
route with no format specified matches any request's format. This is
contrast to the previous behavior, where a route without a specified
format would match requests regardless of their format or whether one
was specified.
This commit also changes the following:
* The return type of the 'content_type' method of 'Request' is now
'Option<ContentType>'.
* The 'ContentType' request guard forwards when the request has no
specified ContentType.
* The 'add_header' and 'replace_header' methods take the header
argument generically.
Closes#120.
* Add content-type responsers for JSON, HTML, and plain text.
* Use content-type responders in content_type example.
* Conditionally create Request `from` HypRequest.
* Clean-up dispatching and handling in main rocket.
* Change Level enum to Logging Level and reexport.
* Allow users to set logging level before launch.
* Fix content_type example error handling.
* Percent decode params when user requests `String`.
