Remove decryptURL: from song

Don’t need it. If we need to later on, it’ll go in Pandora.m. Uses Pandora
crypt functions and I want to introduce different device credentials
accessible through Pandora.device property. There isn't an easy way to get the
decryption password into Song.m, so this is going to go. :-)
Flatten the Pandora API into Pandora.m

This will make it easy to modify device credentials.
Move Pandora.m into Pandora/

It’s not a Model (oops, my bad :-) )
Rename class SearchResult to PandoraSearchResult
Fiddle Crypt.h crypto functions to use any old key

I fixed the namings. PandoraEncrypt and PandoraDecrypt are
**not** symmetrical so they shouldn’t use very similar nomenclature.
Clean up Crypt.[mh] documentation and style
Add Android client credentials & use android by default

(For testing purposes). See credentials here:
https://github.com/pithos/pithos/blob/master/pithos/pandora/data.py#L1

Please note that this define spaghetti is temporary until
a better solution is devised.