Updated resources (css, js, images)

- Updated codemirror
- Set up haskell source highlighting with codemirror
- Changed logo
Implimented haskell evaluation and added some problems
Improved code validation

- special function (validate-code)
- prohibit imports
Allow to fill in the blank or define it

User can either write a definition for function named `__` or simply
write an expression wich will substitute "__" in the tests.
Changed the help page
Changed 404 image
Updated main page
Enabled safe-haskell
Fixed main page
4Clojure -> 4Haskell; cleaned problems