All @example and <doc:example> using IFRAME

Implements the issue (idea) proposed in https://github.com/m7r/grunt-ngdocs/issues/128

I know this is too much of a breaking change, but I'm just going to leave this here for future reference.

I basically ported the code in https://github.com/qbein/grunt-ngdocs/commit/12a4d41865ad5ce8b813aedd8ff40a4f9a03b8bc with some minor changes, including making <doc:example> work exactly like @example, including being aware of `height` and `deps`.