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
Update webp to upstream head
Update project to comply with Xcode 5
Fix long running requests
Merge pull request #479 from longlongjump/master

Fix long running requests
Fix xcode project settings
Merge branch 'master' of https://github.com/rs/SDWebImage