Rur-ple: double the number of languages

While I haven't had time to do programming in the past few months, some Rur-ple users have been busy and submitted new material. The upcoming 1.0 release of Rur-ple will include 3 new languages [Turkish (including a translation of most lessons), German and Welsh] as well as previously included languages [English (with all lessons), French and Spanish].

Inspired by the user contributions, I spent a few hours today re-writing the localization code. To add a new language now only requires the following:

1. Adding one line in a Python file (translation.py), that specify the language and language code, as well as the name of the .po file.
   
2. Creating a .po file containing the string translations.
3. Creating an html file (rur.htm) used as a Welcome Page in that language.

The new version will also save the last language selected and will use it upon re-starting Rur-ple.