Update setTimeOffset

[0rinsb3lt]

I suggest adding a line to setTimeOffset() so that it changes mServerTime by the offset once the function is called.

Steps:

1. getUnixTime()
2. setTimeOffset(offset)
3. getUnixTime()

Expected Behavior:

• Return unix time + offset

Actual Behavior:

• Return unix time

Changing:
void EasyNTPClient::setTimeOffset (int offset) { this->mOffset = offset; }
to:
void EasyNTPClient::setTimeOffset (int offset) { this->mOffset = offset; this->mServerTime = this->mServerTime + this->mOffset; }