Property getters/setters don't work with `get_or_create`
Closing, because I realized that the underlying problem is with `get` and that trying to do a `get` on a derived key like this makes no dang sense.

This is on version 2.6.3, as installed via OSX's `easy_install` function.

I have a project in which I'd like to automatically derive a human-readable sorting key from a title (or in this case, a band name). I have this code for it (simplified):

```
def to_key(name):
    return re.sub(r'^(a|the) (.*)$', r'\2, \1', name.lower())

class Band(BaseModel):
    @property
    def name(self):
        return self.display_name

    @name.setter
    def name(self, value):
        self.display_name = value
        self.sort_key = to_key(value)

    sort_key = CharField(unique=True)
    display_name = CharField(unique=True)
```

This works fine with the model constructor:

```
>>> band = Band(name='The stupidest band name ever')
>>> band.sort_key
'stupidest band name ever, the'
>>> band.display_name
'The stupidest band name ever'
>>> band.save()
1
```

but if I try to use it through `get_or_create` I get an error:

```
>>> band,created = Band.get_or_create(name="The Terrible Trio")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 4145, in get_or_create
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 2740, in get
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 2787, in execute
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 2470, in _execute
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 3199, in execute_sql
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 3048, in __exit__
  File "build/bdist.macosx-10.10-intel/egg/peewee.py", line 3191, in execute_sql
peewee.InterfaceError: Error binding parameter 0 - probably unsupported type.
```

although if I explicitly set the individual properties it's fine:

```
>>> band,created = Band.get_or_create(display_name="Harry",sort_key="Larry")
>>> created
True
>>> band.name
'Harry'
>>> band.display_name
'Harry'
>>> band.sort_key
'Larry'
```

Supporting this isn't a particularly big deal for this project, but it'd be a nice to have.

The obvious question would be how to support ambiguous things like:

```
Band.get_or_create(name="The Sugar Shock")
Band.get_or_create(name="Sugar Shock, The")
```

In this case I'd expect the first one to create a new entry with `display_name='The Sugar Shock'` and the second one to throw an exception because it'd try to create a new entry that violates the uniqueness constraint on `sort_key`, but I'm not sure if that's generally the correct thing to expect (or if this even makes a lick of sense in general).
