Yes there is a way :
Simply go go the App, right click and show package contents.
Go into Application/Contents and open the info.plist with a simple text editor.
Add these Lines under the dict key :

    <key>LSUIElement</key>
    <true/>

Yes, there is a difference. NSUIElement is an old key. Since Apple introduced Launchd services, they use the LSUIElement key. This is also working for Adium, I just tested it.

If It says it can't save the file, simply copy it out of the folder to you desktop, edit it, and move it back into the App.

Keep in Mind, that with the LSUI element, the App is working in the Background. It keeps the menu icon, but your app won't appear in the App-Switcher (CMD+Tab) nor in the Force-Quit Menu.

Here is a Screenshot on how I did it :
![Screenshot Text Wrangler Edit][1]


  [1]: https://i.sstatic.net/9EKNW.png