Support for OR queries

This pull request adds two new helper methods `Sequelize.or` and `Sequelize.and`, which takes an arbitrary amount of arguments and transforms them into AND/OR queries. In order to use them you can pass them to the where clause of finder methods.

The syntax looks like this:

```
this.User.find({
  where: Sequelize.and( Sequelize.or("1=1", "2=2"), Sequelize.or("3=3", "4=4") )
})
```