Support for matching against multiple (or all) fulltext fields

Fixes #11

Adds support for the following syntax:

```php
// Specific fields:
$select->match(array('column1', 'column2'), 'words');

// All fulltext fields:
$select->match('*',  'words');
$select->match(/* falsy */, 'words');
```

Apologies, I wasn't able to check against your test cases due to some incompatibility in the Sphinx config file. What version of Sphinx are you running? I'd be happy to look into that further if I knew how to get that RT index working (I am on `2.1.4-release`).

That said, the changes are minimal and they appear to work as expected.