ENH: Add nice repr for all classes.

This restores behaviors from `mr` where Frames classes display useful info. Example:

    In [4]: v = pims.ImageSequence('/Users/danielallan/image_seq_example/*.jpg')

    In [5]: v
    Out[5]: 
    <Frames>
    Source: /Users/danielallan/Desktop/image_seq_example/*.jpg
    Length: 12 frames
    Frame Shape: 512 x 512
    Pixel Datatype: uint8

I wrote a separate `__repr__` for each class. There is some repeated code, to be sure, but each class should be free to display any info pertinent to its own idiosyncrasies. (And all the base classes have a `__repr__` in case user-defined subclasses do not define their own `__repr__`.)

A very basic test confirms that, for every class, `repr(v)` runs without erroring.