Added signatures forBN_sub_word, BN_is_zero, BN_is_one, BN_is_word,
and BN_is_odd.
Fixed incorrect BN_mod_inverse signature.
Added signature for BN_bn2dec.
Added trait implementations for Zero, One, and fmt::Show.
Added to_dec_str function to BigNum impl.
Use CRYPTO_free to free the result of BN_bn2bin.
Removing unused signatures for the time being.
Removed BN_sub_word signature for the time being.
Merge pull request #55 from cjcole/master

Additions and one bug fix (BN_mod_inverse).