Fix for #597.

If the first error in the location list refers to a file that isn't
loaded, entering the quickfix window would close it.

If `hidden` is set, `:quit`-ing the main file would not close the
quickfix window.
Minor cleanup.

Warning signs should not overwrite error ones.
Use buffers instead of file names when placing signs.
Other minor changes.
Rename signer.vim -> signs.vim.
Minor bug fix.
Make the enabled() method optional.
Addresses #530.

If variable g:syntastic_[filetype]_checker is found in user's config,
g:syntastic_[filetype]_checkers is set and a warning is issued.
Merge remote-tracking branch 'origin/notifiers'

Conflicts:
	autoload/syntastic/util.vim