filling new buffer with 0's and bit order change.

[d085132](https://github.com/mafintosh/BitField/commit/d0851327f7f030a1615e9710f1182afe2b30de23) fixes an issue when using `new BitField(number)` where we needed to fill the node buffer with 0's (contains random garbage otherwise).

[0f05295](https://github.com/mafintosh/BitField/commit/0f05295fc3333165f836ddc9106576a89fc29a94) changes the ordering of bits to left-to-right instead of right-to-left. This makes BitField compliant with the BitTorrent spec (which I'm currently implementing).

Fell free to cherry-pick the first commit if you don't want to change the ordering