Improve OS X build and docs

Improved the README with build requirements and links
Added initial credits for Info box on OS X
Improved Xcode build on OS X, should resolve SDL location better
Conditional compile for OS X to stop SDL setting app icon and
overriding the nicer high res image (may not be wanted on all
platforms)
Merge pull request #9 from andypiper/osx-improve

Improve OS X build and docs