A lot of changes

- New about.php page with credits

- index.php: got new links to «all muscles» (a feature on its way),
«all nerves» and the «about» page

- muscle.php: $tutorial_db -> $db. Some new and changed comments.
Changed the $html placeholder design. Biggest new feature is the
possibility to show a image. Changed a lot of variable names, which
makes more sense. Removed some unnecessary line breaks.

- New nerve.php file. Shows all nerves and details about nerve: Latin
and English name and a list of all muscles innervated by the nerve

- Added images

- style.css: Changed a margin and width of the layout