Removed useless processors and attributes for plugins
Now, plugins can be activated thanks to the ext / dt:ext table attributes.
Configuration renaming and TLD improvement
The TLD has been refactored with the new attributes and all descriptions have been improved for a better content assist. Closes dandelion/issues#8
Some attributes have been renamed for a better consistency :
 - sort => sortable
 - paginate => pageable
 - filter => filterable
 - ajax.processing => feature.processing
Removed unused attributes and corresponding processors (plugins are now enabled thanks to the ext / dt:ext table attribute)
Minor perf improvement  in the TableTag by replacing boxed types by primitives.
Added a missing Thymeleaf processor for the fixedHeader plugin