RFR 8150731: Nashorn JSObject linker should be exposed as a service provider

[Sundararajan Athijegannathan]

http://mail.openjdk.java.net/pipermail/nashorn-dev/2016-March/006026.html

Please review http://cr.openjdk.java.net/~sundar/8150731/webrev.02/

I've update the webrev to use provider clause in module-info.java
instead of the older META-INF/services way of exposing a service.

Also, I've split the test as suggested by Michael Haupt suggested earlier.

Thanks,

-Sundar