Bug 687576 - Add support for LATEX_EXTRA_STYLESHEET

Added the possibility for  LATEX_EXTRA_STYLESHEET analogous to  HTML_EXTRA_STYLESHEET. Special attention has been paid to the extension as ".sty" is automatically added by the \usepackage
Removed some comment statements.
Merge pull request #253 from albert-github/feature/bug_687576

Bug 687576 - Add support for LATEX_EXTRA_STYLESHEET