[fix] issue#11

This pull request is fix for the issue#11. The issue was caused by CORE_REGISTER dictionary, which had been expanded to include all available registers but getRegister was not updated. 

getRegister will display only r0 to r16 registers. There's a new method for cortex_m class, which displays register name. 

The debug output was
DEBUG:root:GDB reg: 16 = 0x01
now is
DEBUG:root:GDB reg: pc = 0x01

Tested with KL25Z, K64F, LPC1768 targets.

Regards,
0xc0170