Introduced WorkerSet. (Was Implemented Process Groups)

BASED ON DISCUSSIONS IN THIS THREAD, ProcessGroup HAS BEEN CHANGED. PLEASE SEE END OF THREAD.

This patch allows grouping workers into separate logical units. 

Among other things it will allow us to create dedicated worker pools for specific tasks, for example out-of-process blocking RDBMS calls.

The available functions and their behavior  are:

- ```DEFPG``` : A global constant. Is the name of the default process group. Process 1 belongs to this process group. 

- ```pg_create(name)``` creates a process group with the requested name.

- ```pg_terminate(name)``` terminates all workers of the specified process group and deletes the group too.

- ```addprocs``` takes in an additional keyword parameter, ```pg=DEFPG```. The processes are added to the specified process group.

- ```procs()``` returns ALL processes,  ```procs(pgname)``` returns processes for the particular process group.

- ```nprocs()``` returns count of ALL processes,  ```nprocs(pgname)``` returns count  for the particular process group.

- ```workers()``` returns workers for ```DEFPG```,  ```workers(pgname)``` returns workers for the particular process group.

- ```nworkers()``` returns count for ```DEFPG```,  ```nworkers(pgname)``` returns count for the particular process group.

- ```pmap_pg``` is like ```pmap```, but only executes on workers belonging to the specified process group (first parameter)

- ```pmap(f, lsts...)``` is equivalent to ```pmap_pg(DEFPG, f, lsts...)```

- ```@parallel``` has a new version with 3 arguments. In this version, the first parameter specifies the process group.

- ```remotecall```, ```remotecall_fetch``` and ```remotecall_wait``` have versions where the first parameter is a process group name (instead of process id). The "next" process id from the group is selected for execution. 

### NOTE:
- All processes connect to each other as before, even across process groups. Full visibility across process groups exist.