Put slave back online automatically, if there's enough disk space again ...

Javadoc improvements inspired by “How to stop passwords used by my scm plugin from appearing the log?” on jenkinsci-dev.

Added callable wrapper to actAsync

make AbstractInterceptorCallableWrapper a static inner class for usability in plugins

Updated/changed German translations

@see hudson.maven.MavenUtil#createEmbedder

extension point to intercept FileCallable.act

[FIXED JENKINS-13202] Try to use NIO.2 (Java 7) methods to work with symlinks.

Conflicts:
	changelog.html

remove empty Maven installations before saving them
Thanks!

Integrated a fix Jesse made in stapler.

(Not doing so for resolveSymlink since it would require an awkward four-way return value:
String, null (not a symlink), IOException, <something else> for Java 5/6.)