Adding support for If-Unmodified-Since

WIP 

This pull request adds support for the If-Unmodified-Since header to the PUT and POST methods.

 - [x] verify If-Unmodified-Since header is always lowercase in the handler
 - [x] document the behavior of If-Unmodified-Since in the README and the gotchas.
 - [x] if the If-Unmodified-Since header is not specified, accept the update unconditionally
 - [x] how do we handle the wrong date format?
 - [x] remove nested conditional [here](https://github.com/drsnyder/poky/pull/9/files#L4R27)

/cc @loganlinn