Repackaged base58 and removed _test

Moved base58 into its own sub-package for easy importing into other tools.

Addresses #7 for you.

Since the name of the package is `bas58`, I'd suggest the name of the function is renamed to simply `Encode`.  That way it removes the stutter.  It would read like `return base58.Encode(b)`.  It still immediately obvious that it's doing base58 encoding.