Cohttp_lwt_unix: expose the ctx type equivalence so that custom resolvers can be built
Add test case for custom resolver
In Cohttp_lwt_unix.Client, add a custom_ctx

This makes either generating a custom Conduit context (for picking
a particular source interface) or a custom resolver (for overriding
how to resolver URIs for requests) more easy
More ocamldoc for request and response functors
Add `Cohttp.Connection.compare` to make ordering of connections possible
Improve Cohttp_lwt_unix ocamldoc
Tidy up CHANGES
add copyright to test
Use `foo_of_bar` convention rather than `bar_to_foo` in Auth

suggested by @rgrinberg to be more consistent with rest of Cohttp
Rename `Cohttp.Transfer.encoding_to_string` to `string_of_encoding`
for consistency with the rest of Cohttp's APIs.

Continues @rgrinberg's suggestion
Add an example of how to access the underlying Lwt_unix.file_descr

This obviously only applies when using Cohttp_lwt_unix, but it's
very useful for those cases (e.g. #188 requests it)
Improve ocamldoc for Cohttp_lwt_xhr and add CHANGES entry
Move the String_io_lwt module out of the JS backend and into Lwt

It's of general use in other places where we want to buffer up strings