Add support for If-Modified-Since

[lazka]

We mirror some files which get synced by clients regularly (using curl internally). mirrorbits will respond to these requests by redirecting to the best mirror and the mirror will then return 304 because the file hasn't changed.

Since mirrorbits already knows the mtime of the files it could in theory handle If-Modified-Since and return 304 itself.

[ott]

I implemented this in #131.

[elboulangero]

This one can be closed, was done in #169

[lazka]

it's not merged yet :)

[elboulangero]

Indeed, sorry for the confusion

[jbkempf]

#169 is now merged.