Use the real socket if it's not HTTP

HTTPretty is about patching the socket for port 80 and 443 only, so,
others ports may be mocked by another tool.
DNS Client, for exemple should continue to works with the HTTPretty.
Add tests about the RuntimeError issue

when HTTPretty.enable(), we should contine to do DNS query,
like it's done in the
test_httpretty_should_not_raise_on_socket_send_when_uri_not_registered
test.

The test test_httpretty_should_raise_on_socket_send_when_uri_registered
is written to check that the RuntimeError is still raised when
the port is used for http traffic.
Fix tests

Functional test change the content of POTENTIAL_HTTP_PORTS
but don't reset it. Ignore those changes.
Fix Python 3 support
Merge pull request #87 from Gandi/master

Use the real socket if it's not HTTP