Refactor MIHBigInteger#initWithData to use BN_bin2bn instead of BN_mpi2bn

The current `MIHBigInteger#initWithData:` uses `BN_mpi2bn` which which doesn't seem to be the right option when there is `BN_bin2bn`, and so I refactored the method to use latter mentioned function. Also I have added `MIHBigInteger#initWithMpiData:` to add support for `BN_mpi2bn` too.