rework vim plugin to integrate into syntastic plugin

Instead of reinventing on-the-fly code/type checking fsharp-vim
can reuse the existing functionality of the syntastic plugin.
adjust Makefile with new files
<leader>i does type checks only
fix error highlighting
write logfiles in debug mode only
update README
README: autocommand for fsharp files only
close scratch window after leaving insert mode
README: add hint to debug mode
type checking should map to <leader>t instead
Merge pull request #770 from kongo2002/syntastic_refactor

rework vim plugin to integrate into syntastic