Changed hog class with a new optimized version which exploits SSE2 operations (3x speedup in descriptor computation).
Fixed bug multiple definitions of hog (moved hog.hpp to hog.cpp).
Bug fixed in hog.g: clip_ variable changed from bool to float.
Added non-SSE version of the hog code.
Removed some compilation warnings from people module.
Merge remote-tracking branch 'mmunaro/master'

Conflicts:
	people/include/pcl/people/impl/hog.hpp
	people/include/pcl/people/impl/person_classifier.hpp