Completions are basically a oneshot flag, which provide a `wait()` call
that blocks until 'finish()' has been called on the completion.
The nice aspect of completions is that because it is a oneshot the order
does not matter - if 'finish()' is called before 'wait()' then 'wait()'
returns immediately. It makes the logic for waiting until a thread is
done finishing up some work easy to implement.
This adds generic APIs to libqb for handling thread's, mutex's, and
condition variables. On Linux and OSX these are implemented via the ones
provided by pthreads. On Windows they're implemented via the ones
provided by the Win32 API.
For compiling, the code itself is not conditional, but the Makefile
includes logic to decide which implementation to pick.
Note that it would have been nice to simply use std::thread and friends
from C++11, however using them on MinGW appears to be a bit messy. Since
using the Windows ones directly isn't that hard this was an easy compromise.
