lastModifiedAtPath can now be called asynchronously.
directoryExistsAtPath can now be called asynchronously.
Fix: When returning error back to callee it no longer nils out the _lastError before the callee has a chance to read the error. Instead, before the error is returned to the callee, the _lastError is copied and then sent back to the callee.
Updated readme to include command to check if a directory exists