initial pass at supporting version constraints in value_for_platform
use Chef::VersionConstraint, not Gem in value_for_platform
add tests for pessmisitics to value_for_platform
update comment for value_for_platform
raise error if multiple constraints are met
use raise, not fail
add value_for_platform to DOC_CHANGES.md
add md files for pr#1726
Merge pull request #1884 from opscode/lcg/1726

Lcg/1726