Add new option "allow failing build results"

* Add a new Boolean option to XCodeBuilder.java
* Add equivalent checkbox in config.jelly
* Document new option in help-allowFailingBuildResults.html

Signed-off-by: Tim Ekl <lithium3141@gmail.com>
New option is a Boolean, not a boolean

* Check for null and compare boolean values in XcodeBuilder.java

Signed-off-by: Tim Ekl <lithium3141@gmail.com>
Merge pull request #25 from lithium3141/allow-failing-builds

Add new option "allow failing build results"