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.
Those options are target specific. They are not required for any other target so I would even define GDBServer constructor like this:
```
class GDBServer(threading.Thread):
    """
    This class start a GDB server listening a gdb connection on a specific port.
    It implements the RSP (Remote Serial Protocol).
    """
    def __init__(self, board, port_urlWSS, options = {}):
```
?