Key generation from secret: handle failure of point multiplication

Not all secrets produce a valid public key. If the BIGNUM derived
from the secret is not less than p for the secp256k1 curve then
point multiplication will fail, and no public key can be derived
from the secret.
Merge pull request #15 from runeksvendsen/master

Key generation from secret: handle failure of point multiplication
Improve handling of hex strings

Renamed x() lx() and added separate x()/b2x() functions for going
to/from big-endian data to handle both cases.

Also changed CScript() to display scripts data elements as binary -
closer to the RPC interface.
Fix Py3 incompatibility in CBase58Data
Improve CBitcoinAddress and related syntax

Now you can just write CBitcoinAddress('37k7toV1Nv4DfmQbmZ8KuZDQCYK9x5KpzP')
Merge remote-tracking branch 'upstream/master' into pythonize

* upstream/master:
  Key generation from secret: handle failure of point multiplication