Added changelog and increased version to 0.15.4

Ignore-this: 594864e7508214960fe54d110edd333d

darcs-hash:20100818123659-58252-2bf023d92d753c78f69d6537b14bd76116b5178b
Fixed parsing the number of messages for message summary payload

Ignore-this: b7b8b4024b8a4ee1b55616c57b250697

darcs-hash:20100823100520-56b4d-cc87e89d8e915c59ec80fde0da56723184c46482