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.
@Yangqing 
I personally think that this utility does not belong to `caffe.bin` since currently that utility deals with the network (training, testing, etc.) while this utility simply prepares images into certain format.

However, I can integrate this change into `caffe.bin` if you insist.