Update books.html

Added new book in German language. (TODO: still need to upload cover image)
Create einfuehrung-ins-programmieren-mit-processing.jpg

Cover of »Einführung ins Programmieren mit Processing«, a German textbook published in Aug 2013
Delete einfuehrung-ins-programmieren-mit-processing.jpg
Create einfuehrung-ins-programmieren-mit-processing.jpg
Delete einfuehrung-ins-programmieren-mit-processing.jpg
Create einfuehrung-ins-programmieren-mit-processing.jpg
Delete einfuehrung-ins-programmieren-mit-processing.jpg
Update books.html
Update books.html: inserted new textbook in German language
Merge pull request #250 from matwolf/master

Update books.html: added new textbook in German language