Added globalConditions to schema options

This allows you to apply a fixed condition to every mongoldb query.

Example:
var userSchema = new Schema({
    globalConditions: {
        _type: 'user',
    },
    collection: 'accounts',
});

var adminSchema = new Schema({
    globalConditions: {
        _type: 'admin',
    },
    collection: 'accounts',
});

var accountSchema = new Schema({
    collection: 'accounts',
});

This way you can easily manage single table inheritance.