Add from_signature constructor to VerifyingKey

[ecdsa]

This constructor creates a VerifyingKey from a signature.
It is currently implemented separately in Electrum, see:
https://github.com/spesmilo/electrum/blob/master/lib/bitcoin.py#L468
I guess it would make sense to add it to python-ecdsa instead

[coveralls]

Coverage decreased (-0.87%) to 88.43% when pulling 8a7d343 on ecdsa:fromsig into 5a6fc04 on warner:master.

[tomato42]

from_signature definitely needs test coverage, I'm not sure if modular_sqrt should be placed in separate file, I think it would fit well in numbertheory.py module

[tomato42]

this is now implemented as from_public_key_recovery and from_public_key_recovery_with_digest in ecdsa.keys.VerifyingKey