Add support for binary responses

[breitnw]

Closes #98.

This PR adds a foundation for handling MPD's binary responses. The central change I made was introducing a union data structure (ResponseEntry) which represents a single entry in an MPD response (either a line of text or a block of binary data). [ResponseEntry] ends up replacing [ByteString] in many signatures, most notably those of Parser and mpdSend. I felt that introducing a new type to distinguish text from bytes was the right choice because binary entries must be parsed immediately (in mpdSend) to prevent them from being split across multiple lines.

I've implemented the albumart command (requested in #98), as well as the readpicture and binarylimit commands.

Since I'm new to the project, I'm very open to feedback if you think anything should be improved here! Specifically, I wasn't sure whether it was the right choice to expose the ByteString as a member of AlbumArtChunk, or if some other way of exposing the raw data would be preferred.

[breitnw]

The last two commits add support for the readpicture command, which reads embedded album art from the song's binary tags (as opposed to albumart, which reads album art from a file)

[breitnw]

I changed readPicture to return Maybe AlbumArtChunk (Just <albumart> if a binary tag containing the album art is present, Nothing if no such tag exists). This contrasts with albumArt, which returns an AlbumArtChunk and throws a FileNotFound error if the artwork file is not present. This behavior is consistent with MPD's API, though it is a bit confusing/clunky. Open to suggestions if you think it should be changed.