fix badly encoded non-ascii characters in ampache XMLs

In ampache.php xmlentities seems to break UTF-8 character. e.g. `ö` becomes to `Ã¶` (the single bytes of the multibyte character are encoded separately to xml entities).

Since almost all character are valid in XML I replaced the xmlentities with an version that just change those values that must be replaced. I used the  multbyte function to avoid to tear multibyte chars apart.

There's also a little fix to the album template where the artist name was escaped, which could result in invalid xml.

The fix works for german umlauts and spanish characters in rhythmbox and just player. Non latin characters need to be tested. The fix should work since php 4.3.0.