Fix up some more of the interrupt framework

This (partially) fixes inducer#42, and also improves work on inducer#31.

Now, interrupting with Control-C works when the debugging is started with
set_trace().

The SIGINT handler is enabled via the public function
pudb.set_interrupt_handler.  If you want to be able to enter PuDB from within
your application, just call this function.  By default, it uses SIGINT (i.e.,
Control-C), but you can change it to use a different signal if that one
interferes.

There is still one major issue, which is that when you break in the middle of
an atomic operation, it goes to bdb. After pressing `n` several times you get
to your code, though.

pudb/__init__.py

@@ -116,10 +116,22 @@ def set_trace():
     import sys
     dbg = _get_debugger()
 
+    set_interrupt_handler()
     dbg.set_trace(sys._getframe().f_back)
 
 
+def _interrupt_handler(signum, frame):
+    from pudb import _get_debugger
+    _get_debugger().set_trace(frame)
 
+def set_interrupt_handler(interrupt_signal=None):
+    """
+    Set up an interrupt handler, to activate PuDB when Python receives the
+    signal `interrupt_signal`.  By default it is SIGINT (i.e., Control-C).
+    """
+    import signal
+    interrupt_signal = interrupt_signal or signal.SIGINT
+    signal.signal(interrupt_signal, _interrupt_handler)
 
 def post_mortem(exc_info=None):
     if exc_info is None:

pudb/debugger.py

@@ -5,7 +5,6 @@
 import urwid
 import bdb
 import sys
-import signal
 
 from pudb.settings import load_config, save_config
 CONFIG = load_config()
@@ -30,10 +29,6 @@ def newfunc(*fargs, **fkeywords):
         newfunc.keywords = keywords
         return newfunc
 
-def _interrupt_handler(signum, frame):
-    from pudb import set_trace
-    set_trace()
-
 HELP_TEXT = """\
 Welcome to PuDB, the Python Urwid debugger.
 -------------------------------------------
@@ -317,7 +312,8 @@ def _runscript(self, filename):
             statement = 'execfile( "%s")' % filename
 
         # Set up an interrupt handler
-        signal.signal(signal.SIGINT, _interrupt_handler)
+        from pudb import set_interrupt_handler
+        set_interrupt_handler()
 
         self.run(statement, globals=globals_, locals=locals_)