Improve compatibility of arAkahukuUtil.asyncFetch

Also add newURI and newChannel as like NetUtil.jsm,
and add asyncFetchBinary for our use
Clean up code to load data included in MHT

+ Log detail status of error for files failed (debug)
+ Split complecated arAkahukuMHT.getFileData into
  arAkahukuMHTFileData.asyncGetFileDataFromCacheEntry,
  arAkahukuMHTFileData.asyncGetFileData,
  and arAkahukuUtil.getMIMETypeFromURI
+ Fix load failuare from P2P cache files