Adjust requestFrom, endTransmission, and startTransmissionWire to handle
repeated starts

In the wire library there are several functions where
an unnecessarily complex coding has been used:

  - endTransmission: the availability of data is already
    checked in while(...), therefore need not be checked
    again in the loop.
  - requestFrom: the for-loop has a predefined and fixed
    number of loops. Therefore a check whether the last
    element has been reached is unnecessary and does not
    add any benefit.

Fixes #20

 WIRE class finished. SERCOM class completed. Few correction in HardwareSerial and SERCOMUsart after compilation