Add library loading API

[cash]

It would be great if a plugin could do something like:

elgg_use_library('image_processing');

on pages or actions that require that library rather than doing a include_once in the start.php of the plugin.

Following the class loading design, plugins would have a lib directory that is automatically scanned. In addition, for external libraries (in a vendors directory for example), it would be beneficial to have an explicit registration function:

elgg_register_library('vendors/Zend/ZendMail/ZendPOP3.php');

[cash]

(In [svn:7532]) Fixes #2519 we can now register and load php libraries - blog plugin is using this