Address warnings reported with Peter Hosey's configuration

This commit addresses warnings that are reported when UIImage+PDF is
used in a project with Peter Hosey's warnings configuration. See
https://github.com/boredzo/Warnings-xcconfig and the accompanying blog
post, http://boredzo.org/blog/archives/2009-11-07/warnings, for details
of the configuration
Merge pull request #24 from wilkinsona/warnings

Address warnings reported when using Peter Hosey's configuration