BigInteger: speedup internal mode slightly

Changes should yield a slight speedup per the analysis at https://github.com/phpseclib/phpseclib/pull/317#issuecomment-42122335
Merge pull request #335 from terrafrost/internal-mode-speedup

BigInteger: speedup internal mode slightly

* terrafrost/internal-mode-speedup:
  BigInteger: speedup internal mode slightly