Minor documentation fixes while reading it
fix #14 in a different way:

blueyed spotted this bug which only matters if you change the default
impelmentation. Instead of fixing a bad argument value '' fix the
function input (command line)
support 2 phase setup:
  1) set py path
  2) define everything else

The 1) can be forced by setup_py_path_early setting. Its required to
make YouCompleteMe find UltiSnips as blueyed found out
Use vim.current.buffer instead of vim.buffers[-1]

Fixes #15