Fetchart: add fetching artwork from Wikipedia

[tomjaspers]

Query DBpedia using SPARQL to obtain the filename of the album's cover, as it is used on the relevant Wikipedia article. The direct image url is then obtained using Wikipedia's API.

[Kraymer]

Interesting approach ! Hadn't heard of dbpedia before.

[Kraymer]

Looks good, I just fixed buckets tests that made the Travis build failed.
Could you have a look at the flake8 warnings and add an updated version of the plugin doc to your PR?

[sampsyo]

Wow, very cool! This is a nifty use of dbpedia.

Thanks, @Kraymer, and I agree: this will be totally ready to merge once flake8 and docs are ready.

[brunal]

@Kraymer, given a recent refactorisation your work cannot be cleanly merged any more. Could you update it? You simply have to create a class Wikipedia(ArtSource), turn your function into its get method and the constants into its class variables.

[Kraymer]

@brunal je crois que tu voulais t'adresser à tomjaspers plutôt non ? ;)

[brunal]

Right... @tomjaspers could you update that PR to make it mergeable?

[tomjaspers]

@brunal I was waiting until the refactorisation was merged. I'll do an update tomorrow.

[tomjaspers]

@sampsyo Is this good to be merged, or need I do something else ?

[sampsyo]

Looks good; thank you, @tomjaspers! I blame the week-long delay here on GitHub, who does not notify me when people add new commits to a pull request. Thanks for commenting, which brought my attention back. Merging now.