Use an unsigned long for the baudrate and use Arduino core methods

[romainreignier]

I found the bug that prevents us to use baudrates higher than 19200 (37767 actually). A int was used instead of a unsigned long in the begin() method :)

And I have add some improvements mentioned in the issue #1.

For the value of the timeout, I have used the one found in that library.
Note that I now use micros() instead of millis() to have a more precise timing. But micros() will rollover after only 70 minutes. To avoid that, we can still use millis() that will rollover after approximately 50 days, or a more robust approach would be to use a custom superMicros() using a 64 bits variable like illustrated here (sorry, it is in French).

I have removed the Serial.flush(). If they were needed to clear the RX buffer, we should use the following instead:

while(Serial.read() >= 0);

I have replaced the custom serialRead() method by the Arduino's Serial.readBytes() with a timeout of 0.

Finally, I have replaced the polling of the UCSR0A register by the use of the Arduino's Serial.flush() method, so the library should work with other Arduino architectures like the Arduino DUE and Zero (the build is OK for the Due, I will try to upload it to the DUE tonight).

I manage to have a working communication at 115200 bauds while using qmodbus on a PC and an Arduino UNO.