Add support for lookup_value_regex

No longer a need to subclass `rest_framework.routers.SimpleRouter`. It looks like DRF adopted your get_lookup_regex function and then expanded on it! :smiley:
Internally escapes { and } in `parent_regex` so that they will not cause problems when `.format()` is eventually called on URL templates.
Attempted to update the unit tests to reflect changes in DRF, but the test suite seems a little out of date to begin with.