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
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
Shadow: tweak bias to remove acnee
Use  glPolygonOffset to remove shadow acne

It's a 2002 trick from M. Kilgard
NormalMap should cast shadows too
Temporary comment out exposure/lwhite settings
Reenable ability to change exposure/lwhite.
Honor the per-track flag to disable god rays
Tonemap: Use J. Hable filmic operator

It gives better result on stk enterprise, slightly saturating blacks.
Auria provided tonemap coefficients
Very slight tweak
Missing "track->hasShadows" check
Bugfix : use ambient light from the sun when SH maps are not used
Tigher fit of light extent
Tweak light attenuation.

Doesnt really change the look of the light but reduce their extent.
Merge remote-tracking branch 'origin/master' into new_login