Don't validate points in known_hosts ECDSA keys

Point validation is really expensive and apparently unnecessary when
we already trust the keys in our known_hosts file.

With my current known_hosts file of 657 keys, this reduces the time
taken to complete a remote task in fabric from 24 seconds to 7 seconds.

This requires python-ecdsa >= 0.11.

Closes #270.
Merge branch 'master' into 298-int

Conflicts:
	setup.py
Changelog re #298