Added support for using an array as a value in the findBy criteria.

I added support to use an array as the findBy criteria. It will generate a parenthesis-enclosed, comma-separated list of values.

Example:
```
$mapper->findBy(new Account(), array(
  'id in' => array('0010000000xxxx1', '0010000000xxxx2')
))
```

will result in SOQL:
```
select Id, Name from Account where Id in ('0010000000xxxx1', '0010000000xxxx2')
```