Adding support for queries with multiple ids

This now allows for Increment/Decrement/Find/Aggregation queries for multiple ids at once.

Incrementing / decrementing multiple ids: 
```
Visits.increment(id: [1001, 1002, 2003], timestamp: '2014-01-01')
Visits.decrement(id: [1001, 1002, 1003], timestamp: '2014-01-01')
```

Find for multiple ids at once: 
```
Visits.find(id: [1001, 1002, 2003], year: 2014, month: 1, day: 5)
```

Aggregations across multiple ids: 
```
Visits.aggregate(id: [1001, 1002, 2003], start_date: '2014-01-01', end_date: '2014-01-05')
```

Aggregations across multiple ids w/ data points at the specified interval: 
```
Visits.aggregate(id: [1001, 1002, 2003], start_date: '2014-01-01', end_date: '2014-01-05', interval: :days)
```

Again these are all optimized for performance by limiting the number of requests to Redis (each one of the above queries only makes one request to Redis).