Remove CGLib proxy “magic” in favour of an explicit document method

Previously, the name of an output file was automatically determined
by the name of the method from which the mockMvc.perform call was made.
While (somewhat) clever, to support this for non @Test methods, this
required the use of a CGLib proxy to push and pop some context that
kept track of the name of the current method. This meant it only worked
for non-private methods. It also made it hard to look at the code and
see what would and would not be documented.

This commit updates the library to provide an explicit document method
instead. This method takes the path of an output directory and a
MockMvc ResultActions instance, typically returned from a call to
mockMvc.perform. For example:

document("index",
        this.mockMvc.perform(get("/").accept(MediaTypes.HAL_JSON))
		.andExpect(status().isOk()));

This will perform a GET request to "/", assert that the response is
200 OK and write documentation snippets for the request and response
to a directory named index.
Include language in snippets so that their syntax can be highlighted

cURL requests are marked as being bash and cURL responses are marked as
being http. This allows their syntax to be highlighted in the resulting
HTML.