Added function fieldIn to use the MySQL expression SELECT WHERE field IN...

Added function fieldIn to use the MySQL expression SELECT WHERE field IN (foo, bar, baz).
$field (string) is the field to check if it has rows with the values.
$options (array) are the different options to look for.

Updated the function save() to allow for two booleans arguments set as default to false.
* $return_insert_id = false (boolean) - If set to true, successfully query will return the last successful insert ID instead of boolean.
* $set_insert_id = false (boolean) - If set to true, will set the current object's primary id as the last insert ID. This allows for an object to be inserted an updated within a single script.