Added first method to iterbetter.
The where clause in db queries can be a dict now.

When the where clause is a dict, each key-value pair is make a condition
and joined with AND.

For example:

    >>> db.select("post", where={"category": "popular", "published": True},
    >>> limit=5, _test=True)
    <sql: "SELECT * FROM post WHERE category = 'popular' AND published = 't'
    LIMIT 5">