Use the default value handling of completing-read

On Emacs at least 22.1 and later, completing-read has the 7th argument
DEF which specifies the default value in case the user input is null.
Make use of this argument instead of a home-grown logic to check the
user input later and assign the default value.

This should allow some elisp packages that replace the completion UI,
such as Helm and anything.el, to happily make use of given default
values for their good user experience.
Fixing type by Michael Ernst.
Merge pull request #41 from tadanagao/completion-defaults-001

Use the default value handling of completing-read
Fix the broken default value handling in mew-input-range

This commit fixes a defect in my previous commit 4af7048 which broke the
case where askp is nil.
change the image resizing command

pnmscale was obsoleted by pamscale.
change the image converting command

pngtopnm was obsoleted by pngtopam.
pnmtotiff was extended and renamed to pamtotiff.
pnmtogif is not present but pamtogif is.
Merge pull request #42 from tadanagao/completion-defaults-001

Fix the broken default value handling in mew-input-range
Merge pull request #44 from trueroad/patch-2

change the image converting command
Merge pull request #43 from trueroad/patch-1

change the image resizing command