Its always a good idea to copy blocks

See http://blog.refractalize.org/post/10476042560/copy-vs-retain-for-objective-c-blocks for more info
Fixed a warning occurring because of property attribute change
Analyzer fix about possible retain cycle
Renamed CommandCallback to MGCommandCompleteHandler
Renamed MGBlockCommand to MGAsyncBlockCommand
Update after renaming
Merge pull request #16 from kompozer/develop

Renamed MGBlockCommand and CommandCallback