Convert pdf extension to a service provider

 - Define an 'htmltopdf' provider contract
 - Use TCPDF 6.0.088 internal HTML handling
 - Implement in Publisher and Codex modules
 - Add tecnick.com/tcpdf requirement to main composer.json
 - add isAvailable() method to Provider to simplify conditional flows based on availability of a real service provider (i.e. suppress links when no provider is available.)

Eventually, this extension should be moved out of core, and into a composer installable module repository. The tcpdf package could then be removed from xoops/core, and only installed when needed.
Slight refactoring to reduce complexity
Merge pull request #161 from geekwright/pdf

Convert pdf extension to a service provider
Updating min. PHP to 5.3.7, formatting
Merge pull request #162 from mambax7/master

Updating min. PHP to 5.3.7, formatting
some fixes in Core/Kernel + unit tests files
complete Core tests
complete unit tests Auth Cache Core
complete test files Service Locale Module
Merge branch 'master' of https://github.com/alain91/XoopsCore