Implemented variadic zip_iterator.
Finished docs for parallel algorithms
First pass over N4071, adjusting our implementation to conform to newest specification
Adding parallel algorithms section to docs
Minor documentation teaks.
Merge branch 'master' into zip_iterator

Conflicts:
	hpx/parallel/detail/zip_iterator.hpp
Resolve merge issues
Merge pull request #1178 from STEllAR-GROUP/zip_iterator

Implemented variadic zip_iterator.
Fixed annoying comparison between signed and unsigned warning