img: extract Image to sub-package
img: rename existing Image to Image_OLD

New image object will be developed in increments until it can replace
Image_OLD.
img: add Image.from_file()
img: add Image._from_stream()
img: add image stream recognizer

Image streams are recognized by image_cls_that_can_parse(stream), which
returns the Image subclass that knows how to parse the image format
contained in *stream*.
img: add Jpeg formats to recognizer

Along the way:

* move image_cls_that_can_parse(stream) import into the calling method
to avoid circular import
img: add GIF formats to recognizer
img: add TIFF formats to recognizer
img: add BMP format to recognizer
acpt: add img-characterize-image.feature

Direct image operations are not strictly speaking part of the API, but
an acceptance/integration test will help drive the development and be
handy later as an integration test.
img: add Png.from_stream()
img: add Png._parse_png_headers()

* localize image-related exceptions within the image sub-package to
ease migration when module is reused in python-pptx
img: add StreamReader.read_str()
img: add StreamReader.read_long()
img: add Png._parse_chunk_offsets()
img: add Png._parse_chunks()
img: add Png._parse_IHDR()
img: add Png._parse_pHYs()
img: wire up Png and Image initializers
img: add width, height, and content_type properties