Allow chef_vault_item to fall back to data bags

Fixes #6

This allows cookbooks that use chef_vault_item to work transparently in
situations where chef vault isn't yet in use, such as development/staging
environments, without any changes. This change replaces the 'dev_mode'
attribute that was previously requires for chef_vault_item to use unencrypted
data bags instead.
Merge pull request #9 from opscode-cookbooks/mh/dbag_fallback

Allow chef_vault_item to fall back to data bags