find() should use `ZREVRANGEBYSCORE` instead of `ZRANGEBYSCORE` when min>max.

Hello,
I faced problem with find() when min>max.

Here's the source code.
```
  models.newsfeed_status.find(
      {
        'written_date': {
          min: +since,
          max: '-inf',
          limit: 10
        }
      },
      function(err, ids) {
        console.log(ids);
      }
  );
```

On current version, It results `[]`.
I figured out that nohm uses only `ZRANGEBYSCORE`, and I think the proper action is using `ZREVRANGEBYSCORE` only when min>max.
And, to be fully functional, I implemented `endpoint` option to `find()`'s search parameter. 

```
models.newsfeed_status.find(
      {
        'written_date': {
          min: +since,
          max: '-inf',
          endpoint: '(]', 
          limit: 10
        }
      },
      function(err, ids) { 
        console.log(ids);
      }
  );
```

It results properly in descending order.

`endpoint` is similar to mathematical expression of interval.
default value is `[]`.

Also I added a test case in `/test/findTests.js` for your convenience.
Please review this.