Support Response Body Example

This adds support for parsing a RAML document that has example
attributes under the body attribute.
I was wondering that myself.  If it's not stripped it always has a dangling new-line on it.  I would be fine adding it to the basic attributes.  Without the strip it breaks the parsing spec for the example listed here because it has that newline:

https://github.com/jpb/raml-rb/pull/2/files#diff-9c0e4dada4fd4ed16a92d0bdff734e7eR20

Want me to add it to the basic attributes?