standardizing on 72x72 DPI for all .png files

Adium is stupid and doesn't understand how to scale DPI correctly. If we
standardize on 72x72 DPI, then emoticons render correctly.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Merge pull request #7 from tycho/master

standardizing on 72x72 DPI for all .png files