Beta 9: Support for resuming downloads, faster operation queue

Included in Beta 9 is one big feature and one big performance improvement:

• New support for resuming canceled downloads. If you call cancelTaskWithUUID:storeResumeData: you can specify that any data already downloaded be stored in the memory cache. If you later attempt to download the image at the URL again, it will resume where it left off. (#331) Caveats:
  • Resume data is only stored in the memory cache, not the disk cache
  • Your server must support resuming downloads by responding to requests with the Accept-Range header == bytes and providing either an ETag header or a Last-Modified header.
• Performance switch to PINOperationQueue from NSOperationQueue which turned out to take a lot of the time PINRemoteImage used on the main thread. (#338, #326, #329)