Updated python/incpy.py to be usable independantly of vim.

python/incpy.py:
    Renamed incpy.vimspawn to incpy.spawn and removed it's dependence on incpy.buffer
    Added the ability for incpy.spawn to capture a program's stderr via a different function.
    Added minimal documentation to incpy.spawn

plugin/incpy.vim:
    Updated plugin/incpy.vim to call the correct python function.