Added carthage support for accessibility core

[a-25]

Added Carthage support for AccessibilitySnapshotCore framework.
Closes #130

[NickEntin]

Thanks for adding this @a-25! Can we add a CI job to build this? I want to make sure this doesn't regress, especially as Carthage isn't used for the sample project or other CI jobs.

[a-25]

Hello @NickEntin .
I added a job, could I do anything to check it? Seems like it should run any time when others do but I cannot see it in the list.

[NickEntin]

Looks great, thanks @a-25! I got the CI job added so it will run now. One comment about adding the license to the header and we're good. I'll go ahead and merge it.