_.filter can now accept an array of callbacks

For example:
```
var collection = {
    john: {age: 40, gender: 'male', skills: ['jump', 'swim']},
    david: {age: 28, gender: 'male', skills: ['jump', 'run']},
    mary: {age: 33, gender: 'female', skills: ['run', 'swim']},
    jessica: {age: 26, gender: 'female', skills: ['run', 'swim', 'jump']}
}

var filter1 = function(val, key, coll){
    return _.contains(key, 'a');
}

var filter2 = function(val, key, coll){
    return val.age > 30;
}

_.filter(collection, [filter1, {skills:['swim']}]); //returns [mary, jessica]
_.filter(collection, [filter1, filter2]); //returns [mary]
_.filter(collection, [filter2, {gender: 'male'}]); //returns [john]
```