Fixes `-Werror=odr`

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.