Simplify the formula, and I guess this makes the formula more
reliable.  Imagine somebody configured `buffered_before_play` larger
than 25%; then the decoder would be woken up all the time.  This
doesn't seem logical.  On the other hand, it's easy to understand that
the decoder should be woken up below 75% buffer fill.

Calculate the value only once.

Shouldn't ever happen, but who knows...

The `SliceBuffer` constructor can throw.

This adds support for the full set of MPD 0.21 filter types.

That way, each plugin can decide to implement it better.

Apparently, Travis-CI will never upgrade its ancient Ubuntu Trusty,
so I have to get Boost from somewhere else.

Thanks to Martin Hierholzer for his PPA.

This passes the new `sort` and `window` attributes to the mounted
database.