RFR: String.join(), StringJoiner additions

[Jim Gish]

Please review 
http://cr.openjdk.java.net/~jgish/Bugs-5015163-7175206-7172553/ 
<http://cr.openjdk.java.net/%7Ejgish/Bugs-5015163-7175206-7172553/>

These are changes that we made in lambda that we're now bringing into JDK8.

I've made a couple of additions - making StringJoiner final and adding a 
couple of constructors to set the emptyOutput chars.

Thanks,
    Jim

-- 
Jim Gish | Consulting Member of Technical Staff | +1.781.442.0304
Oracle Java Platform Group | Core Libraries Team
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