Added in functionality to connect to DB2 over ODBC

Added db2 connectivity functionality to the framework.  Modifications to database.php configuration file should be as follows (added to connections array): -

```php
'db2' => array(
	'driver'	=> 'db2',
	'host'      => 'localhost',
	'database'  => 'database',
	'username'  => 'root',
	'password'  => '',
	'prefix'    => '',
	'library'	=> '' // optional
),
```
Note:  This is designed to connect to a remote DB2 installation.  If you're deploying on AS400/IBMi, your PHP installation with be compiled with ibm_db2 drivers - that connectivity is coming shortly.

pdo_odbc must be enabled in your PHP installation.  For Windows users, this will work just fine.  OSX users... well, we have to EAD.  I'm still working on getting this working with PHP recompiling.  Currently using MAMP and PHP 5.4.4 (thinking unixODBC with Macports might be the best option, although I was hoping to make it easier for n00bs than