Add grooveshark backend

[camilonova]

Adding support for https://github.com/camilonova/mopidy-grooveshark

[coveralls]

Coverage remained the same at 32.69% when pulling dde939b on camilonova:add-grooveshark into eb58e74 on woutervanwijk:develop.

[kingosticks]

Shame it doesn't have a font awesome icon yet (FortAwesome/Font-Awesome#2844)

[kingosticks]

Thanks.

[camilonova]

@kingosticks shame is they have a PipedPier icon and not a grooveshark one ;)

I have tried to understand where this icon is shown, can you tell?

[kingosticks]

to the left of the track in pretty much all the pages. helps people tell which backend their search result came from, for example.

[kingosticks]

And in fact, not just tracks, pretty much anything.

[camilonova]

Saw it.

[kingosticks]

But not when you get the tracks from the Browse page (see #99)

[kingosticks]

actually, that is complete rubbish (I should have checked), it works perfectly from the Browse page too.

[kingosticks]

Grooveshark has since closed down. Might as well remove this now.