Make build work (#546)

The mbedtls example has caused problem in the main build a number of
times. By making it a standalone `cargo new --bin`, we can keep it in
the source tree as a good example but avoid having it break the main
build.

Also, fix some clippy lints.

examples/mbedtls/README.md

@@ -0,0 +1,9 @@
+mbedtls-example
+===============
+
+To run this example you need to `cd` into the directory and use `cargo run`.
+
+```
+cd ureq/exampled/mbedtls
+cargo run
+```