Added custom mac addresses + dhcp renew, longer tcp ack retransmit timeout, improved error handling.

Added support for:
+ begin(byte *mac) {...} lets you specify your own MAC address [begin() is still supported]
+ Fix to not send "Content-Type" in GET requests.
+ Close connection if data size error, so it can restart gracefully.
+ Added maintain() method that should be called periodically to refresh DHCP settings.