Add evhttp_connection_set_family() to set addrinfo->family for DNS requests

This is useful if you want to avoid extra dns requests.
test: add family argument for http_connection_test_()
test: add regress for evhttp_connection_set_family() with AF_INET and AF_UNSPEC
test/http: add regress test for set family to AF_INET6
Merge pull request #118 from azat/http-forward-family-to-bufferevent

Add evhttp_connection_set_family() to set addrinfo->family for DNS requests