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

Close #5

The missing break in every case will have MODE0 always selected

SAMD21G18A doesn't operate correctly with clock dividers lower than 4

Signed-off-by: Thibaut VIARD <thibaut.viard@atmel.com>