[gwt-contrib] Update the npapi plugin to support OSX. (issue1036801)

[conroy]

Reviewers: jat, fabiomfv,

Description:
Update the npapi plugin to support OSX.

Please review this at http://gwt-code-reviews.appspot.com/1036801/show

Affected files:
  M plugins/npapi/Makefile
  M plugins/npapi/manifest-template.json
  A  
plugins/npapi/prebuilt/gwt-dev-plugin/Darwin-gcc3/gwtDev.plugin/Contents/Info.plist
  M plugins/npapi/prebuilt/gwt-dev-plugin/manifest.json


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