Allow plugins to add templates directories

Allow plugins to add templates directories with filter
'tl_templates_directories'.  Functions changed :
tl_get_default_loop_templates(), tl_get_loop_templates(),
tl_display_loop().
tl_locate_template() is no more necessary since priority is defined in
function tl_get_loop_templates().
fixes for my previous commit (Ulrich's questions)

Fixed stuff related to your questions in tl-functions.php + added entry
in FAQ about templates directories
WordPress coding standards
Merge pull request #42 from gordielachance/master

Allow plugins to add templates directories with filter