Added user-defined columns & filters

This pull request allows client code to specify additional columns, like this:

```javascript
new WebSQLStore(db, 'user', [{'name': 'first_name'}]);
```

Calls to `Collection.fetch()` can then filter on these columns using arbitrary SQL:

```javascript
var j_users = users.fetch({
  'filters': "first_name > 'J' AND first_name < 'K'"
});
```

or using `_.where()` syntax (which is converted to a `WHERE` clause under the hood):

```javascript
var jareds = users.fetch({
  'filters': {'first_name': 'Jared'}
});
```