XStreamExcludeEmpty annotation added

[Enigo]

Hi, the use-case for this newly created annotation is quite simple: omit the Collection, Map and String fields in case if they are empty. Like in jackson this can be done using JsonInclude.NOT_EMPTY source annotation. I understand, that the changes are rather significant (after all, it's a new API), so I'm very open to discussion and improvements. If this change looks good, I'll write more test, I promise)

[coveralls]

Coverage increased (+0.02%) to 77.974% when pulling 6c5c55f375bc9b98e783da861942e06258d98709 on Enigo:master into c7bcb6620462d5a2670d331ec3504bf8bc2880be on x-stream:master.