Update CMakeLists.txt

This commit includes checking whether ../data directory (from build directory) and ../../stk-assets directory (from build directory) exists or not. If they don't exist, then it gives error message for the corresponding directory. I have included the code for this on line 320.
Update CMakeLists.txt
Properly add Depth of Field
Add debug sliders for vlj
Use hasShadows flag from Track
Limit the DoF influence
Merge branch 'master' of https://github.com/abhishek1995/stk-code into abhishek1995-master
Let cmake test if stk-assets exists in the expected place. If not,
cmake is aborted with an error message. The option -DCHECK_ASSETS=off
option can be used to disable this test. The latter is also used
for travis.
Make possible to tweak exposure/lwhite in adm