Add "append" mode for convert_imageset.

[tonykcn]

Sometimes, we need to append additional images into lmdb/leveldb, especiallly in fine-tuning stage.

If we construct all data with convert_imageset from ZERO, it's inefficient when the image count is large.

If "append" is set explicitly, new image data are appended after existing rows, no need to touch existing constructed data.

[tonykcn]

Sometimes, we need to append additional images into lmdb/leveldb, especiallly in fine-tuning stage.
If we construct all data with convert_imageset from ZERO, it's inefficient when the image count is large.
If "append" is set explicitly, new image data are appended after existing rows, no need to touch existing constructed data.

@shelhamer , could you help review and merge this change? Any suggestion/comment will be appreciated.

Thanks in advance.