[JENKINS-16215] Avoiding unnecessary downloads in FilePath.installIfNecessaryFrom.

The updated version uses the locally cached timestamp, '.timestamp', to set the
'If-Modified-Since' header. 304 Not Modified responses are dealt with by
aborting the installation.
Merge pull request #660 from larshvile/master

 [FIXED JENKINS-16215] Avoiding unnecessary downloads in FilePath.installIfNecessaryFrom