Adding stencil example walk through.  The goal of this section is to
take a serial example and turn it into a distributed one.
Airplane work, most of example 2
Explanation of examples 1 and 2
Starting to implement N3960
More work on foreach, started to add tests, non-functional, still
for_each functional
Fix return type handling for for_each
Adding missing file
Adding more tests, handle exceptions
Adding tests for for_each_n
Adding transform, started to work on tests
Adding explanation of example 3 and 4
Fixing exception handling, adding more transform  test
Merge remote-tracking branch 'upstream/master' into n3960
Adding transform_binary tests, fixing other tests
Removing old for_each
Made execution policy selection more pretty
Properly handling sequential execution policy
More test fixes
Adding constexpr in some places, fixing for_each_n and corresponding test