Additional file support (zlib/uncompressed, gzip)

Split up decoding and uncompression in differenent functions.
Added support for base64/uncompressed.

Updated the example in the README

The new io crate is used in flate so tiled has been updated to use it as
well.

Thew new io crate gets rid of methods like read_u32_le, so we have to do
it ourselves.

Something weird happens with slicing syntax. from_str changed to parse

Closure syntax has changed. from_str is now parse.

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()