records: exposing record metadata revisions

* Adds view tests for record revision API
  (addresses inveniosoftware/invenio#1990)

Signed-off-by: Bogdan Kulynych <bogdan.kulynych@gmail.com>
Instead of pass-ing a not-implemented test -- which would falsely report it as OK -- you can rename the function to `def FIXME_test_foo()` and decorate it with `@nottest` which would make the test not to run.  This is the technique we've been typically using to mark a test case as "yes I have to get back here one day to fix it".  See for example https://github.com/inveniosoftware/invenio/blob/master/modules/websearch/lib/websearch_regression_tests.py#L4192  (Another technique would be to `@skip` tests which we couldn't do in *maint/master* due to lower version of Python, but which we can do in *next/pu*.)