Emacs 23 compatibility fix

Check for font-lock-refresh-defaults before calling.
Consistency in GFM naming
quiet byte-compiler
enforce use spaces for indentation
Correct comment
Don't attempt numeric comparison on nil

If there was no match to a nearby header then level will be nil, and
(> nil 1) will raise an error.  I'm not convinced that simply ignoring
the prefix arg in this case is the right thing to do, but it's
certainly the expedient thing to do.
DWIM when point is on an setext header

If point is at the "|" in:

    asdfasfasfd|sadfasdfasdf
    =======

then I want markdown-insert-header-dwim to turn it into:

    asdfasfasfd|sadfasdfasdf
    ========================

Checking thing-at-point-looking-at first makes this possible.  I'm not
sure if the rest of this function's features still make sense after
this change, but from what I can tell so far, it is indeed "doing what
I mean."