Show full filename in tooltips for Python profiles.

Only works with some graphviz output formats (e.g. SVG).

Only supports the 'pstats' input format (mostly because I'm unfamiliar
with the other formats, and because I don't know if the problem is as
pressing with those as it is with Python profiles).

Fixes jrfonseca#14.

gprof2dot.py

@@ -217,6 +217,7 @@ def __init__(self, id, name):
         self.called = None
         self.weight = None
         self.cycle = None
+        self.filename = None
 
     def add_call(self, call):
         if call.callee_id in self.calls:
@@ -2583,6 +2584,7 @@ def get_function(self, key):
             id = len(self.function_ids)
             name = self.get_function_name(key)
             function = Function(id, name)
+            function.filename = key[0]
             self.profile.functions[id] = function
             self.function_ids[key] = id
         else:
@@ -2917,6 +2919,7 @@ def graph(self, profile, theme):
                 color = self.color(theme.node_bgcolor(weight)), 
                 fontcolor = self.color(theme.node_fgcolor(weight)), 
                 fontsize = "%.2f" % theme.node_fontsize(weight),
+                tooltip = function.filename,
             )
 
             for _, call in sorted_iteritems(function.calls):
@@ -2981,6 +2984,8 @@ def attr_list(self, attrs):
         self.write(' [')
         first = True
         for name, value in sorted_iteritems(attrs):
+            if value is None:
+                continue
             if first:
                 first = False
             else: