For std::runtime_error.  See http://bugs.musicpd.org/view.php?id=4509

Mapped to "=" in the MPD protocol.  This is the first operator,
initially supporting value matches in the MPD protocol.

Replaces the sticker_domain for Error::domain.

Call sqlite3_reset() only after a sqlite3_stmt* has been used, not
before.  Assume it is already in reusable state.

SQLite 3.7.3 is the version shipped in Debian Squeeze (oldstable);
should be old enough to be the required version.  Drop the
compatibility #ifdef.

For free().

Don't depend on the "simple" database plugin.  This fixes an assertion
failure / crash and allows using stickers with other plugins.