switch truncate_digest_for_ecdsa to context manager

Also adds a docstring to fix #1335
Merge pull request #1349 from reaperhulk/more-ctx-mger

switch truncate_digest_for_ecdsa to context manager
Added missing vector file
Merge pull request #1350 from michael-hart/ec_vectors

Added missing vector file
Part 1 of rebase, with corrections for pep8
Added all changes lost in merge reset
Corrected tests
Added code and tests of EC public keys
flake8
Merge branch 'public_key_pem' of https://github.com/michael-hart/cryptography into public_key_pem

Conflicts:
	cryptography/hazmat/backends/openssl/backend.py
	tests/hazmat/primitives/test_serialization.py