Add image queue length limit

By default, set a limit of 50 images that can be added to the internal
image queue.  Once there are 50 images, the oldest images will be
dropped from the queue to make room for new images.

Adding a limit prevents the module from using all system memory if
continuous capture is enabled, but the next() method is called slower
than new images arrive.

Update docstrings for current arugments

Closes #162