Custom index definitions

[Tim020]

Allow custom indexes to be specified to be created by Couch Potato, for example:

class UserModel(couch_potato.Model):
    __bucket__ = "test"
    __key_generator__ = KeyGenerator("User::{name}")

    name = String()
    age = Integer()

    indexes = [Index("my_index", name, age), Index("my_index_2", age)]