common.hpp: If using std, explicitly define the func_lib namespace so we can pull in _1, _2, etc. Refs #1847.
Since func_lib now pulls in _1, _2, etc., we don't need using placeholders. Refs #1847.
CHANGELOG: Added issue 1847 for func_lib::_1, etc. Refs #1847.
Merge branch 'issue/1847-put-_1-etc-in-func_lib'. Refs #1847.
http://redmine.named-data.net/issues/1847