Add support for source IP address to SimpleAsyncHTTPClient.

Add support for source IP address to SimpleAsyncHTTPClient.

Extend support for HTTPRequest's network_interface parameter to
SimpleAsyncHTTPClient. This enables binding to specific local IP and
enables connecting to WebSockets via a specific IP address.

To use it you create a HTTPRequest with network_interface parameter set
to local IP address to be used as source IP. websocket_connect then
passes the request to SimpleAsyncHTTPClient.

Note that even though the parameter name is `network_interface`, the
CurlAsyncHTTPClient may support (OS dependant) passing in interface
name, resolvable hostname or IP address, while this patch only adds
IP address support to SimpleAsyncHTTPClient.