EZP-21245: Make it easier to register eZXML custom tags in main XSL stylesheet

Added default implementation.
EZP-21245: Added semantic configuration
EZP-21245: Changed semantic config to be under fieldtypes.ezxml.custom_tags
EZP-21245: Set getXSLTProcessor() to protected + registered PHP functions
EZP-21245: Fixed unit tests
Merge pull request #456 from lolautruche/easierCustomTags

Implemented EZP-21245: Make it easier to register eZXML custom tags in main XSL stylesheet