Disabled use of std::async<> on GCC 4.6.
Merge branch 'master' of github.com:BoostGSoC/boost.afio into more_docs_improvements
Added use example for sync, read, write, truncate.

Added new helper function make_async_data_op_req().
Added new unit test testing that async_data_op_req's metaprogramming works correctly.