Use gflags to parse command line arguments for convert_imageset

In the dev branch, many new options have been added to the utility convert_imageset, which makes the old style parsing tedious and the usage hard to remember. This PR aims to replace positional argument parsing with `gflags`, now that it becomes a dependency.
I am OK both ways. Also, I don't have a strong opinion against default arguments although I personally like everything to be with gflags. It makes bookkeeping easier in the future (as we have already experienced pain of unnamed arguments).