Implemented random triplet batching

Handling #43 (closed)

I've implemented 2 steps out 3.

The triplet selection I'll implement later; #44 (closed) is the priority now.

Do you have some time to review it @amohammadi ?

Thanks

bob/learn/tensorflow/utils/util.py

@@ -62,7 +62,7 @@ def create_mnist_tfrecord(tfrecords_filename, data, labels, n_samples=6000):
         img_raw = img.tostring()
         feature = {'data': _bytes_feature(img_raw),
                    'label': _int64_feature(labels[i]),
-                   'key': _bytes_feature("")
+                   'key': _bytes_feature(b'-')
                    }
 
         example = tf.train.Example(features=tf.train.Features(feature=feature))