core: harmonizes the data type for the process ID

Instead of using arbitrary integer types use pid_t as provided by the
toolchain.

Closes #210.
tbh, my creative memory injected something like this into my virtual ears when I wrote that line http://www.youtube.com/watch?v=N03VuEite3I (i.e. a weird mixup of Babylon Zoos Spaceman and The Prodigys Out of Space)... but I read the above and went forth and openend https://github.com/RIOT-OS/RIOT/issues/1439 to fix this later.
And if I had thunk of Bif Naked, I would not have had that Boomtown Boys mix in mind (i.e. http://www.youtube.com/watch?v=oO_cGwnT-TQ).