HTTP authentication flag for basic/digest HTTP authentication.

Review: https://reviews.apache.org/r/22222
Minor style cleanups and some additional comments.
Split version where N tokens can be specified.

Updated 'split' function in stout/strings so we can choose how many
tokens we want to retreive.

Review: https://reviews.apache.org/r/22825
Simplified strings::split when 'n' != None.