add Basic Auth.
set ENABLE_AUTH flag to enable it.

refer to https://github.com/siklol/Wikitten/commit/c0ce80d07f9b1bf40de96c3e4c51354d28298f53
and https://github.com/siklol/Wikitten/commit/740a161ddc28ed87628d26a5edc6106d90c784b4
Merge pull request #7 from tamano/AddBasicAuth

add Basic Auth.