added g:syntastic_check_on_bufwrite

To have the possibility to switch off the auto command to check at
BufWritePost. To me it is kind of enable or disable syntastic but still
have the choice to check manually.

### Update ###
not stopping the autocmd clearing the loclist is doing what I want
in my rc I use
<code>
:nnoremap \<F6\> :let g:syntastic_check_on_bufwrite != g:syntastic_check_on_bufwrite \<bar\> :SyntasticCheck \<CR\>
</code>
to toggle