Ability to add table_prefix

[Chao Wang]

For instance:

class User(Model):
    table_prefix = 't_'

makes t_user

class MyUser(Model):
   table_prefix = 't_'

makes t_my_user

class User(Model):
  table_prefix = 't_'
  table_name = 'user'

makes `t_user'

class User(Model):
  table_name = 'user'

makes user

And the default to None

class User(Model)
  pass

makes user

To add table_prefix to all tables:

class BaseModel(Model):
    table_prefix = 't_'

class User(BaseModel):
    pass    # user

# or just set the attribute to Model
Model.table_prefix = 't_'

class User(Model):
    pass  # user

[Chao Wang]

done