Make the async Client default (#626)

The previously default Client is moved to `reqwest::blocking`, while the
async client becomes the main API.

Closes #622

examples/blocking.rs

@@ -0,0 +1,19 @@
+//! `cargo run --example blocking`
+#![deny(warnings)]
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::init();
+
+    println!("GET https://www.rust-lang.org");
+
+    let mut res = reqwest::blocking::get("https://www.rust-lang.org/")?;
+
+    println!("Status: {}", res.status());
+    println!("Headers:\n{:?}", res.headers());
+
+    // copy the response body directly to stdout
+    res.copy_to(&mut std::io::stdout())?;
+
+    println!("\n\nDone.");
+    Ok(())
+}