Optimized unit constructor on Numeric class

Hi,

We noticed some performance issues when working with large numbers of integers. We tracked the issue down to the `#method_missing` implementation defined on the `Numeric` class in this gem.

This patch replaces the `#method_missing` implementation by defining the constructors directly on `Numeric`.

On a side note, many of the specs are failing even on a clean master branch. Perhaps they need to be brought up to date?

Thanks!