A lot fixes for NRF51822. Make pyOCD usable with nRF51 CPU.

- add options for gdb_server.py: SoftDevice flash protection, don't break at hard fault handler, correct resume procedure, add extra monitor command.
- mask interrupt when stepping. Required for NRF51. Otherwise GDB server can't step if any interrupt occured during halted state. In case of nrf51 with enabled BLE it happens all the time.
- fix nrf51 reset(), resetStopResume() functions.
- add nrf51 specific gdb_server server.
Hi martin,

I think the reset isn't needed here, as I have removed it from the parent class cortexm in commit https://github.com/mbedmicro/pyOCD/commit/0fcaa56380091bcdaf781dc43070c004e7d56bb0. No bug was reported since that.