Unstable builds should not fail when "Execute script only if build succeeds" is set to True.

I noticed that if "Execute script only if build succeeds" is set to true and the build is unstable, the whole build fails. I don't think that is right.

If the build is unstable it should just skip the whole PostBuildScript part, and that should be it. 

it's a minimal patch, I just changed 
```java
if (scriptOnlyIfSuccess && build.getResult().isWorseThan(Result.SUCCESS)) {
...
return false;
``` 
to
```java
if (scriptOnlyIfSuccess && build.getResult().isWorseThan(Result.SUCCESS)) {
...
return true;
```
(I also added a minimal README)