Hi, thanks for the library!

I was playing around with it and noticed some odd behaviour. Calling setBit(n, true) will set bit n counting from the least significant bit upwards (as expected), but getBit(n) will look in a different place.

var BitMask = require("bit-mask");
var mask = new BitMask("0");

mask.setBit(3, true);
console.log(mask.bits()); // "1000"
console.log('3: ' + mask.getBit(3)); // "3: false"
console.log('7: ' + mask.getBit(7)); // "7: true"

It looks like the logic of getBithas been tuned quite specifically for the file permission case, while also counting bits in the reverse order, and the position of the "0" bit depends on which base was used when initializing the mask, which in the case of file permissions (base 8 with 3 digits) happens to work:

This should probably be changed to mirror how setBit works, with bit 0 being the rightmost, least significant bit.