Fix omnisharp-start-omnisharp-server function on 24.4

When I updated to `emacs 24.4` could't run omnisharp server within emacs. I checked the command that was pushed to shell and it was like

```
mono \~/.omnisharp/OmniSharp/bin/Debug/OmniSharp.exe -s path/to/project
```

And the shell returns error with message:

```
Cannot open assembly '~/.omnisharp/OmniSharp/bin/Debug/OmniSharp.exe': No such file or directory.
```

So looks like the problem is with escaping tilde. I am not sure if my solution is good enough to be merged into master, but it works for me.

What I did is created new function `shell-quote-argument-safe`, which doesn't escape `~` on emacs of version >= 24.4. I just made a copy of `shell-quote-argument` function with simple modification. 

Actually I haven't checked this function on emacs < 24.4, but it should work. 

In case you know better solution for this issue – please use it :)