Simple `<l:task post="true" href="someurl"/>` triggers post request as expected
but then proceeds to href as normal link to. If the URL accepts GET then request
is sent twice it if does not after POSTing on background user get
'POST is required' error.

- Typo
- Print error combined
- Set result only when phase like in other code parts.
- Test for expected error message in log

- Print publisher display name instead of class, so user can understand what publisher in UI was used and failed.
- Setter for artifact archiver shows how it handle status
- Minor typos
- Added test for stacktraces in build log

Reflecting more code review comments from
https://github.com/jenkinsci/jenkins/pull/1679

- Jesse wants to keep the variable pointing to 1.6
- Update executable-war so that it fails gracefully if run with Java6 or
  earlier.
- CI build fails with VerifyError due to a regression in JDK.
  To support wide range of development environment, just cut off the
  verifier.
- Have enforcer require 1.7 although javac fails in a kinda obvious way.

Given the minefield that is databinding, that seems prudent.

newInstancesFromHeteroList should recover gracefully from the case that both kind and $class are missing.

As the first step of Java7 migration, ship Java7 class files without
requiring Java7 runtime.

```
WARNING: Caught exception evaluating: executor.progress in /ajaxExecutors. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
...
	at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.ArithmeticException: / by zero
	at hudson.model.Executor.getProgress(Executor.java:649)
	... 154 more
```