First attempt at a barrier implementation. Is completely untested, but it compiles.
Cleaned up the set of call() overloads;

Added when_all for a single async_io_op.

Added new test case to test the new barrier() implementation. Unfortunately it triggers the memory corrupting packaged_task in Boost v1.53 :(
gcov build now defines NDEBUG and therefore code not needed for coverage analysis.
Added new issue to remind me to add mingw32 build support so we can get Travis CI doing Windows unit testing via wine heh heh.
* Issue b11/9f9 (Add a std::vector<async_io_op> barrier() which completes each of its inputs when and only when all of its inputs become ready.) fixed.