Introduce a new operation abstraction to reduce coupling with MockMvc

Prior to this commit, MockMvc-classes were used throughout Spring REST
Docs. For example, each Snippet was called with an MvcResult. This
has proven problematic for a few reasons:

1. The MockMvc APIs aren't very amenable to modifying a request or
   response before it's documented. This caused the existing support
   for response modification to rely on CGLib proxies and method
   interceptors. A similary complex solution for request modifiction
   would also have been necessary.
2. Things are harder to reason about than they need to be as the
   MockHttpServletRequest and MockHttpServletResponse classes
   expose more than is required when generating API documentation.
3. Supporting other test frameworks, such as Rest Assured, is hard

This commit introduces a new Operation abstract that encapsulates
all of the information required to document the request that was sent
and the response that was received when performing an operation on a
RESTful service. The new abstraction uses types from Spring's Web
support, such as HttpHeaders, RequestMethod, and MediaType, but does
not rely on MockMvc.

Closes gh-108

docs/src/test/java/com/example/PathParameters.java

@@ -17,9 +17,9 @@
 package com.example;
 
 import static org.springframework.restdocs.RestDocumentation.document;
+import static org.springframework.restdocs.RestDocumentationRequestBuilders.get;
 import static org.springframework.restdocs.request.RequestDocumentation.parameterWithName;
 import static org.springframework.restdocs.request.RequestDocumentation.pathParameters;
-import static org.springframework.restdocs.RestDocumentationRequestBuilders.get;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
 import org.springframework.test.web.servlet.MockMvc;