bugfix: #[wasm_import_module] as #[link(...)]

[cgmcintyr]

closes #2

[cgmcintyr]

I'm quite new to rust, so apologies if this isn't the right way to use the link attribute.

[cgmcintyr]

The force pushes were a result of me thinking #![feature(wasm_import_module)] should be left in lib.rs. However this broke the build.

I've now tested with stable (rustc 1.31.0 (abe02cefd 2018-12-04)) and cwa-rs compiles fine without #![feature(wasm_import_module)] being present :)