Fix for extending classes in a subdirectory (e.g. drivers)

Hopefully I've got the workflow right this time -- created a branch for the patch.

Using this fix for getting the $baseclass of an extended class, items such as extended drivers (which always have subdirectories) are loaded properly. You can for example extend the Session class by placing the extended class in /application/libraries/Session/MY_Session.php

Here is a sort of description of the problem and updated logic in `Loader->_ci_load_class()` using pseudo code.

Let's say we are loading the "`session`" driver, and we have overloaded the session class in `/application/libraries/Session/MY_Session.php`

The Loader has passed "`Session/session`" to `Loader->_ci_load_class()` through `Loader->library()` when we called `Loader->driver( 'session' )` . It does this because drivers must be in subdirectories.

Start out by setting `$class`, we don't have ".php" in it or any trailing slashes, so it stays as `Session/session`

Check for a `$subdir`, and there is one, "`Session`", so store it, and remove it from the class name so the class name is now just "`session`" and the $subdir is "`Session`".

Capitalize the class name first letter so it's "`Session`"

Check to see if there is a custom application class:
`$subclass = APPPATH.'libraries/'.$subdir.config_item('subclass_prefix').$class.'.php';`

So we are checking: `/application/libraries/Session/MY_Session.php`, which is where we have saved the file, so it exists.

Now check for a corresponding CI base class: **(This is where the previous version fails)**
`$baseclass = BASEPATH.'libraries/'.$subdir.$class.'.php';`

$baseclass becomes "`/system/libraries/Session/Session.php`" which is correct with the update. However, the previous version leaves out `$subdir`:

`$baseclass = BASEPATH.'libraries/'.$class.'.php';`

So it would be impossible for it to match the base class to the application class in the case of a driver/subdirectory! "`/system/libraries/Session.php`" of course does not exist.

From there, it would throw the "`Unable to load the requested class`" error and the application is hosed.

The update is just adds the concatenation of `$subdir` to the `$baseclass` path so that it can have a chance to find application driver subclasses.