avoid crashes by nil-ing out unsafe properties

both the delegate and dataSource properties of UITableView are unsafe
unretained, therefore we should set them to nil upon deallocation
see this StackOverflow question:
http://stackoverflow.com/questions/26147424/crash-in-uitableview-sending-message-to-deallocated-uiviewcontroller
Merge pull request #169 from CalQL8ed-K-OS/zero_out_unsafe_props

avoid crashes by nil-ing out unsafe properties
Merge branch 'master' of https://github.com/xmartlabs/XLForm