.all() improvements

[voidus]

Semantically, .all() should not be a method of the CountryInfo object, since that one represents the information for a specific country.

Instead, it should probably be a global function. While we're at it, I'd also say it should return a list of CountryInfo objects. If a user wants to access a specific one, they can create a CountryInfo object, so fast access isn't a big issue, and it would hide the json structure from library users.

Implementation-wise, this would be easier after #40