This fixes issue #59 and optimize the way the prescaler is found.
Furthermore non needed instructions are not repeated.

The shortcut has some issues:

- sometimes it fails when sending an odd number of bytes (may be
  due to memory alignment?)
- the data pointer should point to "stable" data (and this is not
  guaranteed by caller, it may be some sort of temporary buffer)
- the SRAM is not guaranteed to start at 0x20000000

All the above problems must be properly fixed before reenabling
this part

* fix in_endpoint buffer size (was too big)
* use the same EP for CDC_ENDPOINT_OUT and CDC_ENDPOINT_IN (different buffers)
* fix CDC_ENDPOINT_IN ep type

This way all the 64 bytes available can be filled up

Fixes #30

See #30

calling digitalWrite with any value different from 0 will end
in performing digitalWrite(HIGH)

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

Fix for deadlock conditions after i2c bus errors