Implement "count" method

This is useful when dealing with queries related to pagination where you want to retrieve data with a limit and offset, but also need to know the total number of records that match the query, e.g.

``` php
$sth = $db->prepare("SELECT `foo` FROM `bar` WHERE `baz` = 'qux' LIMIT 0, 10");
$sth->fetchAll();
```

The above gives up to 10 results and no indications of how many results there are in total that match the query.

It would be great to have a method `count`, which effectively is able to interpret the existing query, remove the "LIMIT" claus and give the number of total available records:

``` php
$sth->count();
```

This in effect would execute another query:

``` php
$sth = $db->prepare("SELECT COUNT(1) FROM `bar` WHERE `baz` = 'qux'");
```

The only drawback is that this would introduce SQL parser dependency.