Introduce custom keyboard shortcuts for SizeUp
Update SizeUp namespace to be consistent with other settings

All other custom settings in private.xml are prefixed with a namespace of
`com.jasonrudolph`. Update the SizeUp settings to follow the same convention.
Update screenshots to include SizeUp Mode

- Replace the existing KeyRemap4MacBook "Change Key" screenshot with a handful
  of smaller screenshots
- Add a screenshot for SizeUp mode
Identify specific OS X version used for testing
Document SizeUp mode in README
Merge pull request #4 from jasonrudolph/size-up-mappings

Introduce custom keyboard shortcuts for SizeUp