Bug fixes

• Initialize lock in YYWebImageOperation
• Remove "image/webp" from HTTP header if there's no webp subspec

Bug fixes

• Potentially problem when system load high #79
• Avoid duplicate symbol error when use -all_load link flag YYImage#24

Bug fixes

• Remove unnecessary code #78
• Potentially problem when system load high #79

Bug fixes

• Potentially crash when access view's property in background thread
• Remove YYDispatchQueuePool due to dynamic library compile error

First release, stable API

• Bug fixes
• Swift api compatibility
• Bump the libwebp framework to 0.5