Endian Trait Derivation

This provides a custom derive for the Endian trait on structs.

It currently only works on normal structs with named fields. It does not work on tuple structs or enums. I would like to expand it to reach these before I consider this ready for a 1.0 release.

```rust extern crate endian_trait;

[macro_use]

extern crate endiantraitderive;

[derive(Endian)]

struct Example { a: i32, // others } ```