Added keyboard shortcuts, printer selection and fixed some issues I run into

Hello,

- I have updated README.md to document the issues I run into and what I believe is helpful for others not to have the same trouble.
- I have added searching for 'enscript' inside /usr/bin and /usr/local/bin because printing was not possible on my OS X  10.8 without absolute paths.
- Added printer selection inside the user specific settings based on the output of 'lpstat -d' and 'lpstat -a'. This is described in README.md as well.
- Keyboard shortcuts are set to COMMAND+ALT+P on OS X and CTRL+ALT+P for the rest