Stop passing engine into EngineVersion

Fixes #95
Merge pull request #118 from johnwards/fix/rds_engine_version

Stop passing engine into EngineVersion
providers/aws: allow "modifying" as pending state for db instances

fixes #119
update changelog
providers/aws: require description in security groups. fixes #123
update changelog
website: make ssh_key configuration for droplet clearer
website: document security group description requirement
providers/aws: db instances now depend on db security groups
providers/aws: remove password from resource diff to not trigger changes

cc/ @mitchellh

This one caught me a bit, and I'm not sure if there's another AttrType
that could be useful here – perhaps a AttrTypeHidden or something? I
don't know...this feels sloppy to have in the diff func.
providers/cloudflare: add bins

close #126
Fix example digital ocean image name
Merge pull request #128 from nrempel/patch-1

Fix example digital ocean image name
Merge branch 'master' of https://github.com/hashicorp/terraform
Adds more TODOs to implement later on