Implement zero value logic for BitSet so it can be more easily
incorporated into other structs.

While here, simplify New() to take exactly one parameter. Since
BitSet now supports a zero-valued struct, there is no need for
a no-arg version of New().
Fix documentation to match source comments for zero valued usage.
Merge pull request #3 from tvierling/zerovalue

Implement zero value logic for BitSet so it can be more easily incorporated into other structs.