[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
[JENKINS-16215] : changelog
Added serialVersionUID to all Callables
to build=bauen, but for 'Build' there's no good short german translation
Minor German translation updates
[FIXED JENKINS-16319] Stream ordering problem prevented SecretRewriter from working on Windows.
[JENKINS-16273] Improved instructions for passing -jnlpCredentials.
First, display instructions when the user has CONNECT, not necessarily ADMINISTER.
Second, when anonymous users cannot CONNECT, show how to use -jnlpCredentials
(and do not bother showing javaws, since it does not work in this case).
[JENKINS-16444] Diagnostics useful for this and similar exceptions during Jelly evaluation.
[FIXED JENKINS-16444] NPE showEvenIfUnstableOption when first adding trigger section.
At this time the target project is unset so it has no known type and we just have to assume the threshold is valid.
[JENKINS-13541] Noting.
[FIXED JENKINS-14529] BindException when using --daemon with JMX.
creating an RC branch
the trunk is toward 1.501-SNAPSHOT