Compatibility Fix for insert-mode Tab mapping

I cleaned up my code a little and included a check if the respective mapping is already in use (e.g. by supertab). If there is a previous mapping, nothing will be done. This could be solved by delegating the mapping via ```maparg()``` but I was unable to do it due to my limited knowledge of vimscript and other internals. Feel free to include this if you want. Supertab does something similar.