Changing to AppExtensionURLStringKey.
Removed excluded UIActivityTypes.

Because in iOS 8 beta 5 only the 1Password Extension is visible.
Merge pull request #39 from AgileBits/task/remove-excluded-UIActivityTypes

Removed excluded UIActivityTypes.
Fixed problem when an error is returned when not needed.

This should fix issue #41 when an error is returned when cancelling the share sheet or the Extension itself.
Merge pull request #32 from AgileBits/fixed-readme-typo

Changing to AppExtensionURLStringKey.
Added new helper class method to avoid code duplication
Better method name
Making isAppExtensionAvailable as NS_EXTENSION_UNAVAILABLE.
A more elegant way! @roustem++
Using a single error.code is clearer.
Making a few changes in how we handle "Cancelled By User" scenario.
Adding a missing completion handler call (spotted by awesome @radazzouz)
Removing OP prefix from the error codes.
Merge pull request #42 from AgileBits/bugfix/fix-problem-when-an-error-is-returned-when-cancelling

Added a special `AppExtensionErrorCodeCancelledByUser` error code used when the extension was cancelled by the user and the result can be ignored without showing the error message.
Merge pull request #43 from AgileBits/use-ns_extension_unavailable-for-methods-using-uiapplication

Making isAppExtensionAvailable as NS_EXTENSION_UNAVAILABLE.