Adding support for binary (4-bit) to hex character ('0'..'f').
Add support for MAC address print out in hex/colon format.
Improving W5100 Ethernet controller driver.

1. Introducing dev_flush() to remove contents of receiver buffer.
2. Fixing disconnect() so that the socket receiver buffer is flushed. Hardware
does not do this automatically.
3. Introducing get_addr() to allow access to network address and subnet mask
when using DHCP style begin.
4. Cleanup bind() and reduction of code in begin().
Removing RTC user level interrupt handler. Replaced by RTC::Timer.
Several improvement.

1. MAC address print out in hex/colon format.
2. Reuse INET print out functions.
3. Read line from request and print with uptime/request number.