Switch to Result-based API. (#132)

Gets rid of synthetic_error, and makes the various send_* methods return `Result<Response, Error>`.
Introduces a new error type "HTTP", which represents an error due to status codes 4xx or 5xx.
The HTTP error type contains a boxed Response, so users can read the actual response if they want.
Adds an `error_for_status` setting to disable the functionality of treating 4xx and 5xx as errors.
Adds .unwrap() to a lot of tests.

Fixes #128.

README.md

@@ -17,15 +17,12 @@ let resp = ureq::post("https://myapi.example.com/ingest")
     .send_json(serde_json::json!({
         "name": "martin",
         "rust": true
-    }));
+    }))?;
 
 // .ok() tells if response is 200-299.
 if resp.ok() {
   println!("success: {}", resp.into_string()?);
 } else {
-  // This can include errors like failure to parse URL or
-  // connect timeout. They are treated as synthetic
-  // HTTP-level error statuses.
   println!("error {}: {}", resp.status(), resp.into_string()?);
 }
 ```