Update ShareKit.podspec

Adding Imgur to the Podspec.
SHKFormOptionController done method now calls SHKFormOptionControllerCancelEnumerateOptions

In case there is a pending request to update the list of options, but the user selects an option before the request completes, we need to call SHKFormOptionControllerCancelEnumerateOptions just like we do within the cancel method.
Merge pull request #930 from ChristopherDoherty/patch-1

Update ShareKit.podspec
Merge pull request #931 from gragland/patch-1

SHKFormOptionController done method now calls SHKFormOptionControllerCan...
BUGFIX: URLPictureURI is showed for all types of share (also for file shares). Fixes #928