Merge pull request #2 from PeqNP/develop

Call success/failure callbacks on main thread.
Merge pull request #3 from PeqNP/develop

Version 1.1.0
Fix: Return -1 if command failed to get size of file
Fix: Added comment stating directories can not use the SIZE command
New: Added directoryExistsAtPath:
Merge branch 'Buffer' into develop