Clean up Goal model a bit; #141

The Goal model is only needed right now by the test suite, and there
only to ensure that the database is torn down properly. I'm actually a
little nervous having this object around, because the `goals` table
should be considered immutable. Harumph.

gittip/models/goal.py

@@ -9,7 +9,12 @@ class Goal(db.Model):
     id = Column(Integer, nullable=False, primary_key=True)
     ctime = Column(TIMESTAMP(timezone=True), nullable=False)
     mtime = Column(TIMESTAMP(timezone=True), nullable=False, default="now()")
-    participant = Column(Text, ForeignKey("participants.id", onupdate="CASCADE",
-                                     ondelete="RESTRICT"), nullable=False)
-    amount = Column(Numeric(precision=35, scale=2), nullable=False)
+    participant = Column(Text
+                        , ForeignKey( "participants.id"
+                                    , onupdate="CASCADE"
+                                    , ondelete="RESTRICT"
+                                     )
+                        , nullable=False
+                         )
+    goal = Column(Numeric(precision=35, scale=2), nullable=True)