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
make Headers implement Clone