return whatever the definition returns

this can allow method chaining so that if you call a definition to
create a resource, and the definition takes the time to return the
resource as its retval, then you can hang a .not_if off of the
definition (or .only_if/.notifies/.subscribes/etc)
convert to rspec 3
add test for retval from definition
add md file
Merge pull request #2185 from opscode/lcg/definition-retval

return whatever the definition returns