Cleanup.  Deprecate a number of checker variables.

New / refactored deprecation logging functions:
syntastic#log#oneTimeWarn() and syntastic#log#deprecationWarn().
Bug fix: refresh notifiers in BufEnter, rather than BufWinEnter.
Error highlighting cleanup.
Manual: add a note about netrw.
Make syntastic#util#parseShebang() aware of /usr/bin/env.