Replace xml-rs by quick_xml

quick_xml is way faster than xml-rs

Here is an example with a quick atom parser:
    With xml-rs:
        test parse_factorio_atom ... bench:   3,295,678 ns/iter (+/- 165,851)
    With quick_xml:
        test parse_factorio_atom ... bench:     203,215 ns/iter (+/- 13,485)

Unfortunately I had to break the API for this change to happen.
* Element::from_reader now takes `R: BufRead` instead of `R: Read`
* Element::write_to now takes `W: io::Write` instead of `EventWriter<W: Write>`

This migration also allow us to have a write_to function which assumes
we're already in a given namespace (see `write_to_in_namespace`).

src/lib.rs

@@ -64,7 +64,7 @@
 //! minidom = "*"
 //! ```
 
-extern crate xml;
+extern crate quick_xml;
 #[macro_use] extern crate error_chain;
 
 pub mod error;