experiments/stable_sort: Add debug thread trace

If DEBUG > 0, then stable_sort() prints out a character for every
decision point at which it could either execute in parallel or
sequentially. Parallel execution is indicated by the character "p",
sequential execution by "-". Example trace:

    ppppp---ppppp---pp---ppp------------------------------------------
    -----p----p------p---p-------------------------------------------p
    p-------pp----------pp----pp---------------------p--------------p-
    -------------------p------p------------------------------
experiments/stable_sort: Add debug printout macro
experiments/stable_sort: Add C++11 vector backend

The USE_TBB macro can be used to switch between Intel TBB and C++11
thread backends.
Update proposal PDF
experiments/stable_sort: Write python script
log: Add notes for Wednesday
experiments/stable_sort: Fix insertion sort length
proposal: Add a couple of flow diagrams