Fix client example so it compiles

`URL::parse()` needs an `unwrap()`, the `let res = ...` needs a semicolon, and I changed the example to set a real header.
Merge pull request #121 from jkleint/patch-1

Fix client example so it compiles
std::from_str has been moved to std::str
TaskPool updates
Merge pull request #122 from jgillich/str

from_str & TaskPool updates
feat(server): keep-alive!

Internals have been shuffled around such that Request and Reponse are
now given only a mutable reference to the stream, instead of being
allowed to consume it. This allows the server to re-use the streams if
keep-alive is true.

A task pool is used, and the number of the threads can currently be
adjusted by using the `listen_threads()` method on Server.

[breaking-change]