Only stop server if it was started by default

Omnisharp's current setting for `g:Omnisharp_stop_server` asks to stop the server when *any* vim session exits while the server is running.  I use numerous different vim sessions for different tasks, so this nagging quickly becomes annoying.  This change adds a new setting for `g:Omnisharp_stop_server`:

```
" g:Omnisharp_stop_server == 0  :: never stop server
" g:Omnisharp_stop_server == 1  :: always ask
" g:Omnisharp_stop_server == 2  :: stop if this vim started
```

The new setting is also the new default for `g:Omnisharp_stop_server`. It only stops the server if this vim session started it and does not ask.