core: Add RereadConfiguration event.  Signal SIGHUP fires it.

I've tried to take into account everything what was suggested in #105. However,

1. I didn't documented the new '--handle-signals' option in boot.py:352, because I don't think it is "notable".  Probably, I should document it elsewhere.

2. There is no signals.SIGHUP on Windows:
```
    Python 2.7.1 (r271:86832, Nov 27 2010, 18:30:46) [MSC v.1500 32 bit (Intel)] on
    win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import signal
    >>> signal.getsignal(signal.SIGHUP)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'module' object has no attribute 'SIGHUP'
    >>> signal.getsignal(signal.SIGINT)
    <built-in function default_int_handler>
    >>> 
```