Module implementation: adding themeability

Previously released versions of the module merged walking the taxonomy tree with rendering for most situations, leaving little room for themeability.

This has been split in three parts:

• Data is obtained in the new _taxonomy_context_build_tree($context→tid, $base)
• Rendering is performed in the new theme_taxonomy_context_children_list($tid, $base = FALSE)
• A deprecation warning is inserted in the obsolete taxonomy_context_show_children

This has been split in three parts:

• Data is obtained in taxonomy_context_nodes($tid)
• Rendering is performed in the new theme_taxonomy_context_nodes()
• A deprecation warning is inserted in the obsolete taxonomy_context_show_nodes