Rewrite annotations to be more descriptive.

Method level:
 * `@FormUrlEncoded`
 * `@Multipart`

Parameter level:
 * `@Path` - URL path replacement
 * `@Query` - URL query string
 * `@Field` - Form URL-encoded field
 * `@Part` - Multi-part part
 * `@Body` - Single-object body (Old `@SingleEntity`)

This removes support for `@QueryParam`/`@QueryParams` in favor of including constants in the declared URL itself. e.g.,

    @GET("/list?foo=bar")

We also bring in support for method and parameter-level header declarations for fine control.