Log Message:
Update to mpc 0.15

Changes since previous release:
* optimized and simplified the "add" command, don't obtain and print
  song list when adding directories
* same optimization for the "ls" command, which did some very
  expensive underscore matching logic using "listall"
* show MPD error message in "status"
* fixed special character handling in bash completion
* don't fall back to ISO-Latin-1 in unknown locale
* disable character set conversion to/from pipe
* bug fixes