Support for @internal

[Andrewsville]

The @internal tag has two versions:

• The inline version lets the developer mark a certain part of the documentation as internal and (using a configuration option) decide if such parts should belong to the generated documentation or be silently ignored.
• The standard version marks a whole element (class, function, constant) as internal. Using the configuration option one can decide if the documentation of such elements should be generated.

This implementation slightly differs from the phpDocumentor http://manual.phpdoc.org/HTMLSmartyConverter/HandS/phpDocumentor/tutorial_tags.internal.pkg.html.