adding code for version up, as well as fixing issues with norelativenumber change
Use new NumbersRelativeOff 7.3.1115 compatibility.

VIM versions 7.3.1115 and greater's nu and rnu options interact differently.
See https://github.com/myusuf3/numbers.vim/pull/32\#issuecomment-20896836 or
:h number_relativenumber
Major (opinionated) rewrite to do the following:

- Always keep track of state just don't modify nu or rnu options if
  disabled by setting up auto command group and never disabling it.
- Rename and reorder functions based on calling order and necessary
  scope.
- Save user settings during NumbersEnable and reset them when disabled
Fix s:relative_off like myusuf3/numbers.vim#37.
Create and use buffer and filetype blacklists.
Add notice to README.