toService API

[jeremyrsmith]

This commit makes the following changes:

• Enable a new preferred API, toService. This API allows for creating a service from a request. Rather than specifying content up-front, you can create a function Content => Future[Result] using the toService API, which gives a reusable function from a request specification.
• Major refactoring of types. All request types have been unified to a single HTTPRequest type, which is not path-dependent. A ClientRequest type also wraps this with a provided client.
• Functionality is implemented mainly through Filters now, which composes better with the rest of the finagle ecosystem.