EpsImagePlugin.py speed up and some enhancements

The opening of eps files is speed up by eliminating the search for binary data for pixel based eps files. The offset of that data was expensively calculated and never used. The opening speed could be further improved by replacing PSFile with something speedier.

Other changes include:
- CMYK epsfiles are recognized
- Pixel based eps files are rendered at their original size. The scale on load() is ignored.
- Line art eps files are rendered at 600 dpi (72 dpi previously)
   To restore the older behavior: Image.load(scale=1.0)
   To render at other DPI: Image.load(scale=DPI/72.0)
- The file Tests/test_file_eps.py has been adapted to emulate the previous behavior.