Add import functionality

[hult]

Adds a function Anki::apkg_importer.import that imports decks from an apkg file. Added instructions to the readme file.

[leogiertz]

@rkachowski would be fantastic if you could take a look at this. :)

[rkachowski]

Hmm, my email notifications seem to be screwed up. Thanks for mentioning me - I'll look at this asap

[leogiertz]

@rkachowski Do you have the time to look at this? :)