Removed all external dependencies

- Switched from RPIO to RPi.GPIO
- Copied flufl.enum module directly into RPLCD directory

The project is now free from any external dependencies and therefore
much easier to install.
Fixed documentation bug in README
Fixes and documentation related to special characters

- Added tool to display LCD character map
- Added documentation on how to write characters at non-standard
  locations (e.g. umlauts)
- Enhanced ``write_string()`` documentation