started adding at86rf231 driver

at86rf231 TX and RX

driver is using vtimer instead of hwtimer_ functions, TO CHECK

vtimer debug function prototype fix
bugfixes related to size and buffers
added AT86RF231 payload size
bugfixes : #endif was missing, and TRANSCEIVER_AT86RF231 was colliding with another transceiver.
clean up of ifdefs in transceiver module
fix for #117 and position of preprocessor directives
some debug enhancements
removing residual breaks in switch-case statements due to #ifdefs
Implement simple echo request/reply
Set up new directory structure
Move error codes to public includes
Make sixlowpan module more coding conventions compliant
Add documentation group for 6LoWPAN module
Update documentation of ipv6.h
Make usage of IPHC configurable.
added callback registration for sixlowip
added callback for sixlowpan
Refactor mac.h
Refactor lowpan.h
Refactor border.h