cert_reqs accepts strings and numeric constants

by pushing the resolution of the argument down into ssl_wrap_socket the user is
able to alter the VerifiedHTTPSConnection.cert_reqs without in line with the
public API.
Setting it directly to a string wouldn't have worked earlier.

This also allows to use the constants defined in the ssl module as valid
parameter.
ssl_version accepts strings and numeric constants
short name for cert_reqs
short name for ssl_version
add docs for util.resolve_... functions()
fix docstring of util.resolve_cert_reqs()
add test_ssl_unverified_with_ca_certs
resolve ssl_version, cert_reqs in the connection
remove ssl defaults from HTTPSConnectionPool
add test_verified_without_ca_certs()
remove unnecessary lambda
make test_https pep8 clean
add test for ca_certs in util.ssl_wrap_socket
Merge pull request #130 from t-8ch/unify_ssl_api

Unify ssl api.