Adjust requestFrom, endTransmission, and startTransmissionWire to handle
repeated starts

See https://github.com/arduino/Arduino/issues/3614

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

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

The first occurs when starting a read transaction from a slave that doesn't respond. The code would wait until the SB (slave on bus) bit is set in the INTFLAGS register, but when a nack occurs that never happens so we're stuck in an infinite loop. The fix is to also look for the MB flag to be set. If it is, issue a stop condition and return.

The second happens when a bus error (ie, an illegal stop condition) occurs while sending data as a master. In that case we are waiting for the MB (master on bus) flag to be set. When a bus error occurs that never happens, so again we end up in an infinite loop. The fix here is to also look for the BUSERR flag to be set. If it is, return an error condition.

the function returned 1 millisecond after the required time

See https://github.com/arduino/Arduino/pull/2175

Quoting the original commits from Matthijs Kooijman:

    Enable C++11 support

    This uses the gnu++11 standard, which is C++11 with GNU extensions.
    C++11 should be full compatible with the previously used C++98
    standards, so all pre-existing sketches should continue to work.

    Enable C11 support

    This uses the gnu11 standard, which is C11 with GNU extensions.
    Previously, gnu89 was being used, which is pretty ancient by now. C99
    brings some important improvements, some of which were already available
    and used even without this option. C11 is more recent and brings more
    minor improvements. Most notable feature is the static_assert statement,
    allowing checking invariants at compiletime using the full C
    expressions.