Updates & creation should return a resource representation

[timmipetit]

PUTS and POSTS can update fields that weren't parent of the provided parameters (for example: updated_at, created_at, etc). It would be nice of PUTS and POSTS return an (updated) presentation of the resource.
Right now this requires an extra hit to the API.

[nederdirk]

I fixed this issue on our own fork (Recras:sfDoctrineRestGeneratorPlugin, unfortunately not pull-requestable because it includes the cleanup fixes from #31 and our composer files)
I could make another branch that just depends on #31, if you like that.