Features: setting socket memory sizes and skipping read bytes

This pull request adds two features:

1) Setting the RX/TX memory sizes of sockets by using:
```cpp
uint16_t sizes[4] = {8192,0,0,0}; // 8 Kb memory to the first socket
W5100.setRXMemorySizes(sizes);
W5100.setTXMemorySizes(sizes);
````

2) Adding a skip function to allow faster flushing of the input.