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 understand. However using gflags is still better - we are thinking of merging everything into a single caffe binary, and different utilities having different mandatory arguments implemented as argv will still be hard to maintain (also, they will become non-mandatory in the single caffe binary :)).