Ensure open handles lists correctly

The call to `glob` was causing `list`s of files to fail in `open`. This
fixes that issue by looping over the `list`s contents and concatenating
them.

pims/api.py

@@ -162,7 +162,13 @@ def open(sequence, **kwargs):
     >>> frame_count = len(video) # Number of frames in video
     >>> frame_shape = video.frame_shape # Pixel dimensions of video
     """
-    files = glob.glob(sequence)
+    if isinstance(sequence, str):
+        sequence = [sequence]
+
+    files = []
+    for each_seq in sequence:
+        files.extend(glob.glob(sequence))
+
     if len(files) > 1:
         # todo: test if ImageSequence can read the image type,
         #       delegate to subclasses as needed