Add Repr for Goal

[FergusFettes]

Hey, I just used pyminder in a small project I made: https://github.com/fergusFettes/beetimer

Here is one of the things that I thought of when trying it. But maybe there is a reason you didn't have this.

Just makes it much easier to see the goals. Here is what it looks like now. (Before it just told you the class name.)

In [3]: goals = pyminder.get_goals()

In [4]: goals[0]
Out[4]: <Goal: cartwheels-- six cartwheels or other flips per day (Unit: cartwheel-- safe for 2 days)>

In [5]: print(goals[0])
cartwheels
 six cartwheels or other flips per day
 safe for 2 days
 cartwheel