multi-version extensions to the omnibus DSL
add name of software that throws exception for debugging
raising here doesn't work

- we could add a source? method and use that to inspect if there
  is a source and make #source throw an exception but right now
  having an unset source is perfectly acceptable for gem installs
de-opscodification
add deprecation warnings and YARD
change calls that have been deprecated
add README.md section on version overrides
remove distractions from spec tests
linebreak
fix renaming given_version to default_version
Merge pull request #94 from opscode/versioned-dependencies-2

project overrides of software definitions