Add new option "allow failing build results"

xcode-plugin already provides the ability to parse the results of running unit tests on an Xcode project by way of [configuring a test step](https://wiki.jenkins-ci.org/display/JENKINS/Xcode+Plugin#XcodePlugin-Settingupaunitteststep). However, since the tests run as a build step, a failing test result will fail the entire build. This may or may not be the desired behavior.

This pull request provides the ability to ignore an error return code from `xcodebuild` by way of a new option: "Allow failing build results." If checked, xcode-plugin will not treat a non-zero return code from an `xcodebuild` run as a reason to fail the build step.

This can prove very useful for test build steps configured in tandem with Jenkins' "Publish JUnit test result report" post-build action: if there are failing tests, that action will mark the build unstable, rather than failed. This can help give developers an additional level of information about the status of their build.