Fix compatibility with Xcode 6 beta 1

Fixes #39
Merge pull request #40 from nskboy/master

Fix compatibility with Xcode 6 beta 1
Assertion for UIGraphicsBeginImageContextWithOptions NULL return value
[ios-snapshot-test-case] add log of new saved screenshot when recordMode is set
Fixes typo
Merge pull request #44 from SinnerSchraderMobileMirrors/log-new-images

Add log of new saved screenshot when recordMode is on
Further detailed exceptions for zero-sized layers
Merge pull request #43 from Memrise/catch-cgcontext-null

Assertion for UIGraphicsBeginImageContextWithOptions NULL return value