core: harmonizes the data type for the process ID

Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.
ccnl: pass relay_pid as pointer
at86rf231: use radio_packet_length_t
cc110x: use radio_packet_length_t
cc110x_ng: use radio_packet_length_t
cc2420: use radio_packet_length_t
pyterm: do a proper cleanup at SIGINT
pyterm: set default serial port to /dev/ttyUSB0
pyterm: make config file name a variable
pyterm: introduce command line arguments
pyterm: filter for specific output (via regex)
pyterm: ignore lines based on regex
pyterm: prefix pyterm commands with /
pyterm: configure baudrate
pyterm: added twisted based JSON sending
pyterm: fix program exit procedure, disable SIGINT
pyterm: remove unnecessary debug output
pyterm: allow spaces in JSON regex
pyterm: distinguish between int and string in JSON sending
pyterm: use writeSomeData to send data immediately