Add registerPlugin function.

More details: http://developer.longtailvideo.com/trac/browser/sdks/jw6-plugin-sdk/docs/plugins-js.html
Merge branch 'patch-1' of github.com:audetpascale/DefinitelyTyped into audetpascale-patch-1
fix jwplayer/jwplayer.d.ts. add optional parameter hint.
Merge branch 'audetpascale-patch-1' closes #1788