The new Response instance in the `catch` block didn't have the
`command` attribute set, so the error response didn't indicate which
command had failed, which however is required in the MPD protocol.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/628

Clarify that those can't throw, preparing for the next commit.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/42

A first use case for our libchromaprint integration added by commit
30e22b75

Will allow more complex fitler expression, such as negation (#89).

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Add API documentation
Support 64 bit offsets
Use InputStream for all reads

The first step to multi-player support.  Not much, just a dummy
command.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

handle_tagtypes() is also being moved to ClientCommands.cxx, because
that command will be extended to access client-specific settings.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.