Add emacs unsupported image resizing

Emacs unsupported image size is obtained
after converting the image to PNM (pbm) format.
Merge pull request #53 from trueroad/unsupported-image-resize

Add emacs unsupported image resizing
Fix variable mew-image-display-resize ignored

The variable `mew-image-display-resize` was ignored and did not performed.
Merge pull request #54 from trueroad/fix-resize-flag

Fix variable mew-image-display-resize ignored
Add image resizing always flag

variable `mew-image-display-resize-always`
Change the image resizing command line option

pamscale option -xysize is obsoleted by -xyfit.
Merge pull request #55 from trueroad/image-resize-always

Add image resizing always flag