BigInt: add trailing_zeros, trailing_ones, bits

This is only a proposition, see my [comment](https://github.com/JuliaLang/julia/issues/3560#issuecomment-54711882) (issue #3560) for the decision made here (I'm not sure if I should have used WIP, RFC or nothing for such a small PR).
I guess the implementation of `bits` could be made much faster.