fix property and slotted class python snippets

• `rwprop` did not work at all for me (messed up expansion)
  and did not honor `ultisnips_python_quoting_style`.
• slotclass assigns to variables beginning with an underscore
  in its init function and thus the slots have to begin
  with an underscore too.
Merge pull request #88 from kljohann/master

fix property and slotted class python snippets.