Fix coding conventions in sys/shell/commands/sc_cc2420.c
Fix coding conventions in sys/transceiver/transceiver.c
Allow IEEE 802.15.4 addressing types for transceiver

IEEE 802.15.4 has two addressing modes: 16-bit short and the device's
EUI-64. Currently RIOT supports only sending of packets with 16-bit
short addresses via the transceiver interface. This patch allows at
least for the radio chips that support IEEE 802.15.4 to let the
application/upper layer decide which addressing mode to use.

Upper layer implementation will be implemented in follow-up PR to #460
Create transceiver address type

Motivated by the fact that we might want to support hardware addresses
longer than 16-bit one day. This patch makes it easier to change the
size of addresses.
Add getter and setter of long addresses to transceiver
Implement net_if module
Add shell commands for interface configuration
Add test cases for net_if module