Add a json serilizer for ProcSet (to allow monkey patching)

More details here: https://stackoverflow.com/questions/2343535/easiest-way-to-serialize-a-simple-class-object-with-simplejson/50025192#50025192

src/procset.py

@@ -28,7 +28,21 @@ scheduling. Hence, the manipulated intervals can be represented as indexes.
 """
 
 import operator
-
+import json
+
+class JSONEncoder(json.JSONEncoder):
+    """To make ProcSet JSON serializable you have to Monkey patch the json
+    encoder with the folowing code:
+    >>> import json
+    >>> import procset
+    >>> json.JSONEncoder.default = procset.JSONEncoder.default
+    """
+    def default(self, o):
+        """For JSON serialization."""
+        if isinstance(o, ProcSet):
+            return o.__repr__()
+        else:
+            return super(self,o)
 
 class ProcInt(tuple):
     """A ProcInt is a closed interval of non-negative integers."""