DB structure changes + initial image get support

Added tables tests and test_images and a mapping table
map_tests_question for the many-to-many relation that maps test,
test_questions and test_images.

Added images and precalculated (encoded) css files and output sprite
image for them.
Added records in the DB for them manually.
Added user-side files necessary for image getting (imageGetter.js) and a
imageServer.php that serves it's queries.
Added sample client page imgUser.html