Support iterable and array-based OkHttp Part parts.

Also forbid OkHttp Part parts in iterables and arrays when a part name was specified.

Honor backpressure in Observable creation.

Document maps don't allow null anywhere.

Retrofit web crawler example.

Support HttpUrl with @url.

Add support for OkHttp's MultipartBody.Part.

To do this we need to make `@Part`'s value (i.e., name) optional, since `MultipartBody.Part` brings its name.

`MultipartBody.Part` is a replacement for Retrofit 1's `TypedOutput`/`TypedFile` for doing uploads of files with names.

Remove the need for generated accessor methods.

Null check Call.enqueue argument.

Sample for a dynamic base URL.

Drop the mutable base URL.

Doc error. With `encoded=true` will stop default url encoding, but your doc is misleading.

This was unsafe for switching between development and production
backends. This is because requests that are triggered on a UI thread
may be delayed in execution on a background thread. These could cause
requests created for a development environment to be executed against
a production environment, or vice-versa.

Retrofit is the wrong layer for switching backends for load balancing
or proximity. That can happen in an OkHttp interceptor instead.

Doc error. `@QueryMap` with `encoded=true` will stop default url encoding. So, calling with `foo.list(ImmutableMap.of("foo", "foo+bar"))` yields '/search?foo=foo+bar' instead of '/search?foo=foo%2Bbar'.

Combine MethodHandler and RequestFactory into ServiceMethod.

This is once class that handles all of the annotations and binding for
a service method.

Rename RequestFactoryParser to RequestFactory.Builder.

It's not perfectly a builder, but it fits the pattern of embedding the builder
or factory within a type.

Also rename Response.isSuccess() to Response.isSuccessful().

Also rename RequestAction to ParameterAction. These are only used for
request parameters.

Also move all utility methods into the same class, Utils.java.

Forbid null map values, and null maps.

Make build pass on Windows

Note that the callback executor should be used when non-null.