Features/protobuf needs unicode

This is part of a fix for https://bugzilla.mozilla.org/show_bug.cgi?id=958690

This patch must be used with https://github.com/mozilla/cef/pull/3

heka-py-cef now calls the CEF library with an explicit request to return only unicode strings instead of plain Python string objects.  This is required as python-protobuf expects only 7-bit clean ASCII or unicode string types.