Move UltiSnips-related functions to a separate file
Add get_inner_attr function
Add a function to check if YouCompleteMe is available
Update function decorator to work with classes
Add helper classes for implementation of completer functions
Rewrite ros#launch_complete using new complete helpers

Launch files now have semantic completion for:
  * attributes "pkg" and "output"
  * substitution args
    * environment variables
    * find package
    * package relative pathes