restless.models.serialize improvements

Almost a total rewrite, now sports include/exclude field arguments,
ability to serialize any attribute, not just model fields, and
(hopefully) saner way of specifying related models.
add tests for the new serialize functions

We keep the tests for the deprecated variant to be sure we haven't
broken the backwards compatibility.
update docs to describe the new serialize() helper
remove setup_requires (they're only needed for docs/tests)
add testproject/requirements.txt (for tests/docs)
Merge branch 'master' into better-serialize
Merge pull request #7 from dobarkod/better-serialize

Better serialize