Support IN binding

None of the existing PDO implementations support binding an array to "IN" operator. I propose the following syntax:

``` php
$sth = $db->prepare("SELECT 1 FROM `foo` WHERE `name` IN (s:,foo)");
$sth = $db->prepare("SELECT 1 FROM `foo` WHERE `name` IN (:,foo)"); // or without type-hinting
```

In the above example ":," indicates that placeholder value is an array that needs to be imploded.

``` php
$sth->execute(['foo' => ['1', '2', '3']]);
```

Would effectively result in:

``` php
$sth = $db->prepare("SELECT 1 FROM `foo` WHERE `name` IN (?, ?, ?)");
$sth->execute(['1', '2', '3']);
```

Note that it might be that Doll will need to create a new prepared statement in case it is executed multiple times and number of array elements change.