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.

src/test/range.rs

@@ -9,7 +9,8 @@ use super::super::*;
 fn read_range() {
     let resp = get("https://ureq.s3.eu-central-1.amazonaws.com/sherlock.txt")
         .set("Range", "bytes=1000-1999")
-        .call();
+        .call()
+        .unwrap();
     assert_eq!(resp.status(), 206);
     let mut reader = resp.into_reader();
     let mut buf = vec![];