Rename completion function and file, return results as list

`CompleteScript` should be renamed so it is not a global function. The old name still works and just delegates everything to `tmuxcomplete#complete`. I think this should be deprecated.
This also means the script needs to be renamed to `tmuxcomplete.vim`.

I also removed the call to `complete_add` to return the results as list. Will this break anything? It seems to work fine here.

These changes also make the plugin compatible for a NeoComplete omni-completion function.

NeoComplete configuration example (for all filetypes):
```
let g:neocomplete#sources#omni#functions._ = 'tmuxcomplete#complete'
let g:neocomplete#sources#omni#input_patterns['tmuxcomplete#complete'] = '\S\+'
```