Print a message to the console or /tmp when sent a SIGUSR1

Please *do* test this out before merging.

This will print a stacktrace of the currently running code when any
of the salt-minion or salt-master processes are sent a SIGUSR1 signal.
This is for seeing what's going on and to help with debugging.

Here is how I was testing this patch out:

```
# ps -efH | grep salt-mast[e]r
root     12740  2783  1 20:56 pts/0    00:00:00         python /home/jeff/git/salt/scripts/salt-master -l debug
root     12741 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root     12748 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root     12751 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root     12754 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root     12757 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root     12760 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root     12763 12740  0 20:56 pts/0    00:00:00           python /home/jeff/git/salt/scripts/salt-master -l debug
root@desktopmonster:~# kill -SIGUSR1 12740
root@desktopmonster:~# kill -SIGUSR1 12741
```

Minion example:


```
======== Salt Debug Stack Trace =========
  File "/home/jeff/git/salt/scripts/salt-minion", line 18, in <module>
    main()
  File "/home/jeff/git/salt/scripts/salt-minion", line 15, in main
    minion.start()
  File "/home/jeff/git/salt/salt/__init__.py", line 202, in start
    minion.tune_in()
  File "/home/jeff/git/salt/salt/minion.py", line 510, in tune_in
    time.sleep(0.05)
=========================================
```


Master watchdog process example:


```
======== Salt Debug Stack Trace =========
  File "/home/jeff/git/salt/scripts/salt-master", line 17, in <module>
    main()
  File "/home/jeff/git/salt/scripts/salt-master", line 14, in main
    master.start()
  File "/home/jeff/git/salt/salt/__init__.py", line 110, in start
    master.start()
  File "/home/jeff/git/salt/salt/master.py", line 192, in start
    reqserv.run()
  File "/home/jeff/git/salt/salt/master.py", line 304, in run
    self.__bind()
  File "/home/jeff/git/salt/salt/master.py", line 286, in __bind
    zmq.device(zmq.QUEUE, self.clients, self.workers)
=========================================
```

Master worker example:

```
======== Salt Debug Stack Trace =========
  File "/home/jeff/git/salt/scripts/salt-master", line 17, in <module>
    main()
  File "/home/jeff/git/salt/scripts/salt-master", line 14, in main
    master.start()
  File "/home/jeff/git/salt/salt/__init__.py", line 110, in start
    master.start()
  File "/home/jeff/git/salt/salt/master.py", line 158, in start
    clear_old_jobs_proc.start()
  File "/usr/lib/python2.6/multiprocessing/process.py", line 104, in start
    self._popen = Popen(self)
  File "/usr/lib/python2.6/multiprocessing/forking.py", line 99, in __init__
    code = process_obj._bootstrap()
  File "/usr/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
    self.run()
  File "/usr/lib/python2.6/multiprocessing/process.py", line 88, in run
    self._target(*self._args, **self._kwargs)
  File "/home/jeff/git/salt/salt/master.py", line 145, in _clear_old_jobs
    time.sleep(60)
=========================================
```

I'm sure there are places missed as this needs a *lot* more testing,
but for the moment, it "works for me TM". I'd like to test this out on a ```salt-minion``` process right in the middle of a long-running state-highstate just to see what happens, but don't have that setup on this machine at the moment.

Checking the exception for ```errno.EINTR``` as an idea was shamelessly lifted from [this commit](https://github.com/minrk/ipython/commit/379a29008cb614143024772f7146a926ce0e9278) in ipython.

Spin the wheels, test this out, and let me know what you think.