Workaround for #197, informative error message for not supporting JDK 8

[eis]

Uses maven enforcer rule to notify user that JDK 8 is not yet supported (See #197)

[Shusshu]

I haven't tried Java 8 but I would prefer someone to investigate the cause and fix the cause instead of blocking JDK 8 as a "fix" or "workaround"

[eis]

So would I. The informative error message would be there until someone does that.

[Shusshu]

Maybe you can be that guy ? :)

[eis]

It might be the general issue of javadoc tool in JDK 8 being more strict, see http://mail-archives.apache.org/mod_mbox/hadoop-common-issues/201311.mbox/%3CJIRA.12681661.1385586762252.48109.1385753855233@arcas%3E and http://stackoverflow.com/questions/15886209/maven-is-not-working-in-java-8-when-javadoc-tags-are-incomplete. But ok, I'll add that as a comment to the original defect and will close this pull request.

[Shusshu]

Yup it's better as Java 8 isn't released yet, things could change and work on release day (at least we hope)