Include-what-you-use and removed (some few) unnecessary includes.

[fancycode]

Be explicit with the includes and don't rely on "magic" by the compiler or some other files to include stuff.

Should we check this by Travis?

Also what do you think about using a consistent format for includes? Sometimes, headers are references as libde265/filename.h, sometimes only as filename.h (and sometimes both in the same file). For consistency, we should imho always use libde265/filename.h.

[farindk]

Always using libde265/filename.h is fine for me.

Includes can be checked by Traves, but it should be a non-essential test.

[fancycode]

The includes are updated now. I will do the Travis integration in a separate PR but already added the cpplint configuration.

Manual checking can be done using

$ find libde265 -name "*.cc" -o -name "*.h" | sort -u | xargs cpplint.py

Feel free to merge if everything is green.