Updated bit-set crate to version 0.4

[debris]

Can you also release new version - 0.3.1 ?

Thanks! ;)

[debris]

pr has failed on rust 1.3. Is there a reason to keep this crate backwards compatible with such old version of compiler?

[luciusmagn]

Also old bit-set now breaks with recent Cargo versions... in the next update or the one after, libusb will stop working