The variant's file compliance is now determined using a define named

ARDUINO_SAMD_VARIANT_COMPLIANCE

that refer to the minimum core version required for this variant file
to work.

The attribute ATTR_ANALOG is needed to select DAC, so it has been
removed from pins A1..A5 that doesn't have a DAC.

The selection of the correct timer peripheral is now
decided through the PIN_ATTR_{TIMER/TIMER_ALT} attribute.

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