Add ability to add (define) composite index

[socketpair]

Today, you have composite_key -> add UNIQUE check to the table. I did not found in documentation how I can create composite index, which allows duplicate values and allows effective range queries, which is not true for unique checks.

[kozlovsky]

I added composite_index() which works the same way as composite_key(), but defines non-unique index instead:

class Person(db.Entity):
    id = PrimaryKey(int, auto=True)  # primary key
    passport_id = Required(str, unique=True)  # simple unique index
    name = Required(unicode, index=True)  # simple non-unique index
    age = Required(int)
    composite_key(name, age)  # unique composite index
    composite_index(age, name)  # non-unique composite index