The callbacksForURL: method of SDWebImageDownloader now returns a copied array. This should fix the crash in multithread environment such as:
Collection <__NSArrayM: 0x226d9310> was mutated while being enumerated.
Merge pull request #471 from PinchProject/hotfix/fast_enumeration_fix

The callbacksForURL: method in SDWebImageDownloader should return a copy of the mutable array!
Change version to 3.4
Use 'default_subspec' and separate the WebP support to its sub-spec

This removes unnecessary dependency to 'libwebp' for users who don't need WebP support.
Fix sub-spec dependency
Merge pull request #472 from ikesyo/webp-subspec

Use 'default_subspec' and separate the WebP support to its sub-spec