Bugfix: when a packet is sent by the RDC layer, we should count the number of transmissions made because a single callback can pertain to multiple transmissions at the RDC layer.
Merge pull request #609 from adamdunkels/push/csma-fix

CSMA bugfix: count transmissions correctly
Added method to framer that returns the header length if the framer
would create a header with the current packet information.

This allows sicslowpan to calculate the max payload size without
consuming a sequence number or clearing/restoring the packet buffer.