add readable(), writeable() and settings changed callback to USBSerial

[xiongyihui]

For class Serial has readable() and writeable(), add them to class USBSerial, switch between Serial and USBSerial easily.
Add settings changed callback to USBSerial to implement a USB to UART bridge.

#include "mbed.h"
#include "USBSerial.h"

Serial uart(USBTX, USBRX);
USBSerial pc;

void settingsChanged(int baud, int bits, int parity, int stop)                // Called by ISR
{
    const Serial::Parity parityTable[] = {Serial::None, Serial::Odd, Serial::Even, Serial::Forced0, Serial::Forced1};
    if (stop != 2) {
        stop = 1;   // stop bit(s) = 1 or 1.5
    }
    uart.baud(baud);
    uart.format(bits, parityTable[parity], stop);
}

int main()
{
    pc.attach(settingsChanged);
    while (1) {
        while (uart.readable()) {
           pc.putc(uart.getc());
        }     
        while (pc.readable()) {
            uart.putc(pc.getc());
        }
    }
}