Move to three letter ISO codes

This pair should be moved to three letter ISO codes. The name should probably be `apertium-epo-eng`.

The following files (at minimum) will need to be checked:
* `Makefile.am`
* `configure.ac`
* `modes.xml`
* `README`

The pair should also be checked to see if it can be adapted to work with monolingual language packages in [languages/](http://wiki.apertium.org/wiki/languages)
