Add internet media types of netpbm formats

image/x-portable-bitmap pbm
image/x-portable-graymap pgm
image/x-portable-pixmap ppm
image/x-portable-anymap pnm
image/x-portable-arbitrarymap pam
Fix function mew-image-inline-p return value

function mew-image-inline-p may return strings.
It should return value t or nil only.
Merge pull request #50 from trueroad/pbm-type

Add internet media types of netpbm formats
Merge pull request #51 from trueroad/patch-5

Fix function mew-image-inline-p return value