Workaround for a Vim crash.

Older versions of Vim can crash if redraw is called while a popup is
visible.  This commit adds a variable g:syntastic_delayed_redraws that
instructs syntastic to move redraws to a CursorHold / CursorHoldI
handler if redraw is called while pumvisible() is true.
Document g:syntastic_delayed_redraws.  Minor cleanup.
Merge branch 'delayed_redraws'