Add setPoolServerIP and setPoolServerAddress methods

[keriszafir]

add setPoolServerIP (accepts IP address), setPoolServerAddress (domain as well as IP address) methods

Fixes #103

[WhymustIhaveaname]

It seems that

void NTPClient::setPoolServerIP(IPAddress poolServerIP) {
    this->_poolServerIP = poolServerIP;
    this->_poolServerName = NULL;
}

and

void NTPClient::setPoolServerAddress(IPAddress poolServerAddress) {
    this->_poolServerIP = poolServerAddress;
    this->_poolServerName = NULL;
}

are the same except for variable names.

I also added a void setPoolServerIP(IPAddress server_ip); in my PR#102.

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[matthewdwatkins]

It appears that this PR has been held up for 1.5 years because the contributor hasn't signed the CLA. I need this functionality for a current project. Would it be possible for me to resubmit this and get it merged?

[keriszafir]

Of course :)

[matthewdwatkins]

Looks like it's already been resubmitted multiple times it just hasn't been approved yet. Too bad.