Make Error implement Send.

[akosthekiss]

Fixes #33

[akosthekiss]

Note: Travis is reporting the following error:

error: implementing the trait `core::kinds::Send` is not unsafe`

However, my more recent rustc complains if I remove unsafe:

error: the trait `core::kinds::Send` requires an `unsafe impl` declaration

[alexcrichton]

Thanks! I'll wait for this change to hit nightlies to merge.

Could you also move this to the Error type instead? I would expect it to be more appropriately placed there instead of the FFI bindings.

[akosthekiss]

You are right, it's cleaner this way. Thanks.