Missing implementation of __hash__ in Model
I'm using peewee in several of my projects, and I love it. Today I found a weird behaviour of testing whether models already exist in a dict. I asked a question about it on Stackoverflow: http://stackoverflow.com/questions/35826534/object-in-list-behaves-different-from-object-in-dict

[The answer](http://stackoverflow.com/a/35826785/1650012) turns out to be that the object (in this case an instance of a Model which I got from a peewee `select()`) doesn't seem to implement `__hash__` so it instead uses the “bad hash implementation” (which is based on the object id) as the default. I solved my trouble with al little workaround by using `if theObject in list(theDict)`, which makes use of the `__eq__` method instead of the `__hash__` method. 

I just wanted to let you guys know of this small issue though, so when you have time you can check whether you can add an implementation of the `__hash__` method to Model.

Cheers, and thanks for creating peewee!
