Calls editor using `exec` (OSX fix)

Sets `proc.StartInfo.UseShellExecute` to false so that the editor binary is called
directly with `exec` instead of being launched with the system launcher.

I was trying to use this shim to glue Unity and Sublime Text 2 together in OSX. In the original source, the binary and arguments were passed to the OSX `open` command, which interpreted the file:line argument as another command to run. This pull request lets the shim run successfully with a line like `mono UnityExternalScriptEditorHelper.exe /path/to/file.cs;10` at the command prompt.

Unfortunately, Unity for OSX doesn't recognize individual binaries or executable scripts when selecting an external script editor. It requires an app bundle. I wrote a shell script to call mono and this application, then tried many different ways of packaging that script into an app bundle, but the file:line argument never seemed to make it from Unity to the shell script.

So the last mile is still missing. These changes just clear up the errors. :crying_cat_face: Maybe there's a better way to turn this mono app into a bundle.