xml-rs does not seem to track rust nightly on crates.io. These changes
make sure that the master branch of this repo works with rust nightly.
Whilst Rust < 1.0 this will continue.

The crates.io version of this library will continue on a separate branch

Before this commit parse took an EventReader. Change so that it takes a
Buffer

For some reason the rust compiler no longer needs us to give a type hint
for the variables in get_attrs. This makes the macro read a lot nicer
when used.

Newer versions of rust require slicing to be enabled through a crate
attribute to be able to use [] as .as_slice()