Support for UUID in Sqlite3
LGTM, you're too fast!!

As per the docs, UUID is not natively supported in Sqlite3, resulting in an error when you try and use it;

```
  File "/home/vagrant/.virtualenvs/vagrant/lib/python3.4/site-packages/peewee.py", line 1824, in inner
    return self.parse_node(fn(*args, **kwargs))
  File "/home/vagrant/.virtualenvs/vagrant/lib/python3.4/site-packages/peewee.py", line 1852, in _create_table
    columns.append(self.field_definition(field))
  File "/home/vagrant/.virtualenvs/vagrant/lib/python3.4/site-packages/peewee.py", line 1802, in field_definition
    column_type = self.get_column_type(field.get_db_field())
  File "/home/vagrant/.virtualenvs/vagrant/lib/python3.4/site-packages/peewee.py", line 1431, in get_column_type
    return self._field_map[f]
KeyError: 'uuid'
```

This can be easily added by using a `VARCHAR` instead, sure it's not a native column but at least we get something rather than nothing.

Therefore I'd like to propose adding a field mapping into Sqlite3 for 'uuid=VARCHAR'. If I submit a PR, would this be accepted @coleifer?

In the mean time, I was able to get this working with a monkey patch;

```
overrides = peewee.SqliteDatabase.field_overrides.copy()
overrides['uuid'] = 'VARCHAR'
peewee.SqliteDatabase.field_overrides = overrides
```
