Fallback to regular data bag item loading

This could be used for testing purposes where a cookbook project is
run under test-kitchen and there are data bag item fixtures set up.
When the Chef Vault fails to load, we safely fall back to loading a
data bag item of the same name from the bag, presuming this is for
test purposes.
Use dev_mode attribute

Rather than rescuing a load error (which could have false positives),
fall back to loading the data bag item if a specific attribute is set
for development/testing mode.

For an example of this in practice, see .kitchen.yml.
Merge pull request #2 from opscode-cookbooks/jtimberman/fallback-dbi

Fallback to regular data bag item loading