[boost] Adding my sorting algorithm, ska_sort, to boost, attempt 2

Regarding having an extra single-header version: that seems overkill to me. Boost.Sort already claims to have no dependencies on other Boost libraries and is header-only, which should make it extremely easy to include in any project.

"These algorithms do not use any other library or utility. The parallel algorithms need a C++11 compliant compiler."

FYI: Apparently that statement seems to be not quite up to date: According to boostdep, sort depends on config, core, range, serialization and static_assert. However, that seems to be restricted to the spreadsort headers and transitively to the header (pdqsort also needs type traits) Best Mike