Make AWS settings optional.

If a task is run that requires them but they aren't set, the specific
ones that aren't set will be reported in the resulting error.
Update the README to reflect the optional AWS settings.
Make AWS bucket optional for compass-sass.
Merge pull request #17 from Shadowfiend/optional-aws-settings

Make AWS settings optional

AWS settings should now only be needed if you try to run the deploy-related tasks.
Build-related tasks should not require the settings. If you try to run a task that
requires the settings without having set them, you should get error messages
describing which settings you need.