Enabled and improved proxy support

- Allow :proxy as a valid argument
- Proper implementation of proxy, support both SSL and non-SSL
  connections
Include non-default port number in host header

When port is not the default one, the port number needs to be included
in the host header according to RFC 2616.
Ignore both port 80 and 443 from header
Simplified variable name
Merge pull request #2 from nj/nj/host_header_rfc

Include non-default port number in host header
Fixed issue when running multiple calls

If running multiple calls using the same http2 object, it didn't work
out as expected due to << was affecting the :host.
Merge pull request #3 from nj/host_header_rfc_fix

Fixed issue when running multiple calls
Merge pull request #1 from nj/nj/proxy_support

Enabled and improved proxy support