Enable Control-C to pause the debugger

This fixes #42.  Now pressing Control-C while the script is running enters the
debugger in the line that was run (i.e., it does a set_trace()).

Note that this only works if the script was invoked from `pudb` or `python -m
pudb`.  If PuDB was started with `import pudb; pudb.set_trace()`, this does
not work.  But not that even without this patch, Control-C kills Python
entirely in that case (as opposed to entering post-mortem mode).

pudb/debugger.py

@@ -5,6 +5,7 @@
 import urwid
 import bdb
 import sys
+import signal
 
 from pudb.settings import load_config, save_config
 CONFIG = load_config()
@@ -29,6 +30,10 @@ 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.
 -------------------------------------------
@@ -125,8 +130,6 @@ def newfunc(*fargs, **fkeywords):
 OTHER DEALINGS IN THE SOFTWARE.
 """
 
-
-
 # {{{ debugger interface
 
 class Debugger(bdb.Bdb):
@@ -312,6 +315,10 @@ def _runscript(self, filename):
             statement = 'exec(compile(open("%s").read(), "%s", "exec"))' % (filename, filename)
         else:
             statement = 'execfile( "%s")' % filename
+
+        # Set up an interrupt handler
+        signal.signal(signal.SIGINT, _interrupt_handler)
+
         self.run(statement, globals=globals_, locals=locals_)
 
 # }}}