Fix cargo error in bit-set crate by implementing own minimal BitSet

[AzazKamaz]

I have found that newest cargo is not building libusb-rs because it depends on bit-set v0.2.0 that has incorrect Cargo.toml file (dependency without version). Also found pr #15 that have failed building on rust v1.3 because of bit-set update.

So I have written little implementation of bitset (with tests) that is enough for libusb usage. It's even a bit faster than old one (benchmarked with criterion: https://gist.github.com/AzazKamaz/71c0ff5277033ece07f33b70848c309f)