Base58 test cleanup and extracted vectors

This pull request extracts the test vectors out of `test/base58.js` and `test/base58check.js` and puts them inside `test/fixtures`.
This extraction of the test vectors should allow for easier verification against test integrity with the reference client tests: https://github.com/bitcoin/bitcoin/blob/master/src/test/data/base58_encode_decode.json, https://github.com/bitcoin/bitcoin/blob/master/src/test/data/base58_keys_valid.json and https://github.com/bitcoin/bitcoin/blob/master/src/test/data/base58_keys_invalid.json.

This pull request also fixes a minor bug in the `Address` tests which were not properly testing against the malformed data.

Lastly, the `Address` tests now verify against the same malformed base58 as the `base58` tests.
This is arguably a demonstration of how lightweight the `Address.fromBase58Check` wrapper really is, but I couldn't think of a reason why they shouldn't share the same invalid test vectors.