Add available widths and heights

To mitigate DOS attack on exposed API, as described here:
https://github.com/oo12/phpThumbOf/wiki/Thumb-War

Two new system settings are required: 
phpthumbsup.available_widths
phpthumbsup.available_heights

and added to the $config array. is_available_option checks for these, and if not matched in the request the image is not processed for that option.

Note on upgrade, this will prevent processing of images in a site with pre-existing install, but now that the exposed API vulnerability is brought to light it's a pretty important patch, IMHO.

Could wrap the whole thing in a condition that checks for values in those settings, or another setting to turn this functionality on/off...

**Will submit separate PR for build script to add system settiongs