switch truncate_digest_for_ecdsa to context manager

Also adds a docstring to fix #1335
Part 1 of rebase, with corrections for pep8
Merge pull request #1349 from reaperhulk/more-ctx-mger

switch truncate_digest_for_ecdsa to context manager
Added all changes lost in merge reset
Corrected tests
Added missing vector file
fixes #1340 -- use pypy 2.4.0 now that it's out
Merge pull request #1350 from michael-hart/ec_vectors

Added missing vector file
bump pyenv version
bleh update first
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
Merge pull request #1351 from alex/pypy-2.4

fixes #1340 -- use pypy 2.4.0 now that it's out
Remove (apparently) unneeded line
Merge pull request #1354 from alex/cleanup-travis-install

Remove (apparently) unneeded line
Update multibackend.py

Inserted missing newlines