Merge pull request #14 from rampart81/master

#3 Chunked Encoding 지원 추가
Remove OKFileResponse by refactoring.

This patch is mostly contributed by Eun Woo Song. See the comments in
8cbc9c6ea016e25a7baf6eb67073d7f2ca90d7d0.
cache: Support ETag, If-Match and If-None-Match.

Send ETag and handle If-Match and If-None-Match. This solves #15.
Add missed HttpDate.scala.
cache: Add a comment for isModified().
Rewrite README.md