Add continent code(s) assignment

[Bost]

Please pull. (And please note: continent codes form a set, not a list.)

[Bost]

I just discovered the list of transcontinental countries. It's just getting more and more complicated, hmm :( Quite a few corrections has to be done in my PR and also in the datafile on wikipedia)

Another idea: Instead of serving a set of continent codes we can serve a primitive value - just one continent code, by creating "unions" of continents. i.e. if we extend the continent-dictionary by:

    "Eurasia"  : "EUAS",  # Russia
    "Afroasia" : "AFAS",  # Egypt - the only country so far
    "Oceasia"  : "OCAS",  # Indonesia
    "Americas" : "AMER",  # Panama
    # etc.

Let me edit the PR and explore this new idea.