open.vim: Bug fix for error handling code (the "not implemented" message)
xolox#misc#open#file(): Improve documentation & variable names
xolox#misc#open#file(): Instrument with xolox#misc#msg#debug() calls
Properly detect Mac OS X using xolox#misc#os#is_mac()

Related to vim-shell issue #18:
  https://github.com/xolox/vim-shell/issues/18