added client and header timeout.

[csalvo9]

This PR adds 2 new methods:

SetHeaderTimeout(int)
This sets the response header timeout (in seconds) for the http transport.

SetClientTimeout(int)
This sets the command timeout (in seconds). Default is 30 seconds (same as before).

These new timeout values are set in callAPI() during transport / http client init.

This PR also changes the call to callAPI from:
resp, err := callAPI("jsonrpc", url, payload, cli.username, cli.password, .......)
to:
resp, err := cli.callAPI("jsonrpc", url, payload)

[greenpau]

@csalvo9 , thank you for contributing! 👍

Please change

func callAPI(contentType string, url string, payload []byte, username, password string, secure bool) ([]byte, error) {

to:

func (cli *Client) callAPI(contentType string, url string, payload []byte)

This way, instead of:

resp, err := callAPI("jsonrpc", url, payload, cli.username, cli.password, cli.secure, cli.headerTimeout, cli.clientTimeout)

You will do:

resp, err := cli.callAPI("jsonrpc", url, payload)

[greenpau]

@csalvo9 , LGTM 👍

[greenpau]

@csalvo9 , if you have energy, please see if you could resolve #6?