Marking import_warnings deprecated

As suggested by yanick, import_warnings is marked as
deprecated and will emit a warning on its use.

I've added myself as a contributor to the author file for work on
https://github.com/PerlDancer/Dancer/commit/9aee1c73365b876b7110253e6363a5f7a5078c02
Merge pull request #890 from symkat/import_warnings_deprecate

Marking import_warnings deprecated