PLAT-6485/PLAT-6487 - Enhance Result model

Better Javadoc and tests.
Change getFailures() to return ImmutableSet
Use methods generated by Joda-Beans, reverting PLAT-5458.
Use immutable collections
Remove line breaks and duplicates from FailureResult message
PLAT-6485/PLAT-6487 - FixUp from PR
Merge pull request #57 from OpenGamma/topic/PLAT-6485

PLAT-6485/PLAT-6487 - Enhance Result model