Break AppEngine support out of the core.

This isn't the final resting place, but non-standard integrations like this are not going to live in the core.
Add support for iterable headers.
Merge pull request #666 from square/jw/iterable-headers

Add support for iterable headers.
Optional @Field/@FieldMap URL-encoding.
Merge branch 'karlicoss-encode-field'
Merge pull request #636 from square/jw/break-app-engine-out

Break AppEngine support out of the core.