feat(client): add a new Client struct with super powers

- Includes ergonomic traits like IntoUrl and IntoBody, allowing easy
usage.
- Client can have a RedirectPolicy.
- Client can have a SslVerifier.

Updated benchmarks for client. (Disabled rust-http client bench since it
hangs.)

README.md

@@ -42,18 +42,18 @@ Client:
 
 ```rust
 fn main() {
+    // Create a client.
+    let mut client = Client::new();
+
     // Creating an outgoing request.
-    let mut req = Request::get(Url::parse("http://www.gooogle.com/").unwrap()).unwrap();
+    let mut res = client.get("http://www.gooogle.com/")
+        // set a header
+        .header(Connection(vec![Close]))
+        // let 'er go!
+        .send();
 
-    // Setting a header.
-    req.headers_mut().set(Connection(vec![Close]));
-
-    // Start the Request, writing headers and starting streaming.
-    let res = req.start().unwrap()
-        // Send the Request.
-        .send().unwrap()
-        // Read the Response.
-        .read_to_string().unwrap();
+    // Read the Response.
+    let body = res.read_to_string().unwrap();
 
     println!("Response: {}", res);
 }
@@ -64,22 +64,20 @@ fn main() {
 [Client Bench:](./benches/client.rs)
 
 ```
-
 running 3 tests
-test bench_curl  ... bench:    298416 ns/iter (+/- 132455)
-test bench_http  ... bench:    292725 ns/iter (+/- 167575)
-test bench_hyper ... bench:    222819 ns/iter (+/- 86615)
+test bench_curl  ... bench:    400253 ns/iter (+/- 143539)
+test bench_hyper ... bench:    181703 ns/iter (+/- 46529)
 
-test result: ok. 0 passed; 0 failed; 0 ignored; 3 measured
+test result: ok. 0 passed; 0 failed; 0 ignored; 2 measured
 ```
 
 [Mock Client Bench:](./benches/client_mock_tcp.rs)
 
 ```
 running 3 tests
-test bench_mock_curl  ... bench:     25254 ns/iter (+/- 2113)
-test bench_mock_http  ... bench:     43585 ns/iter (+/- 1206)
-test bench_mock_hyper ... bench:     27153 ns/iter (+/- 2227)
+test bench_mock_curl  ... bench:     53987 ns/iter (+/- 1735)
+test bench_mock_http  ... bench:     43569 ns/iter (+/- 1409)
+test bench_mock_hyper ... bench:     20996 ns/iter (+/- 1742)
 
 test result: ok. 0 passed; 0 failed; 0 ignored; 3 measured
 ```