removed CocoaPods install instructions - not yet on CP

clarified 2nd example

added info to README

using constants instead of straight passing response codes in method call

implemented RETR command support

moved stopPassiveServer further up

tidied up with #pragma mark

TYPE command implementation now actually setting transfer mode

clearing passive server when data has been sent

550 error return when failing to read file attributes

removed debug output

implemented MDTM command support

implemented SIZE command support

implemented MLST command support

[DFFileSystemNavigator currentVirtualPath]