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.
@netheril96 I think keeping this a separate utility, that also uses gflags,
makes sense as the caffe tool is for taking actions with nets.

@Yangqing thoughts?

On Sunday, August 10, 2014, netheril96 <notifications@github.com> wrote:

> In tools/convert_imageset.cpp:
>
> >          "Usage:\n"
> > -        "    convert_imageset [-g] ROOTFOLDER/ LISTFILE DB_NAME"
> > -        " RANDOM_SHUFFLE_DATA[0 or 1] DB_BACKEND[leveldb or lmdb]"
> > -        " [resize_height] [resize_width]\n"
> > +        "    convert_imageset [FLAGS] ROOTFOLDER/ LISTFILE DB_NAME\n"
>
> @Yangqing <https://github.com/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.
>
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> <https://github.com/BVLC/caffe/pull/857/files#r16031584>.
>