Native Joomla! Module Cache support

Adding full module caching that respects the global Joomla caching settings is very very easy, just these parameters in the xml and the Joomla core will provide the feature. I tested on 2.5 and 3.0.

Notice two things:

- `cache_time` parameter conflicts with the `cache_time` parameter you already have for the GD image generation cache, i put it anyway two times in the parameters and put a TODO in your xml. You can keep it also conflicting because it seems to not create an issue but you can control the parameter from both tabs, but anyway I hope that after merging this, you will soon add some refactoring and separate the cache_time (that is a parameter name reserved for the native module cache helper) from the internal thumbinail generation cache 


I think this fixes #12