Add a dummy bn_is_zero C dependency to wrap BN_is_zero

This is necessary because on some architectures BN_is_zero
is a CPP macro, so trying to link against it in an `extern "C"`
block causes a linker error.

This also introduces a build command to Cargo to compile
the bn_is_zero wrapper.
Merge pull request #64 from reem/fix/big-num-is-zero

Add a dummy bn_is_zero C dependency to wrap BN_is_zero