Updated plugin config and after_platform_add hook

Changed after_platform_add hook to use cordova to install plugin
instead of build.  Removed gap:plugin tags in config.xml (unused with
cordova).
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Background:
Since upgrading to latest version of build.phonegap and cordova,
cordova or “plugman” should be used to handle plugin installation:
http://docs.phonegap.com/en/3.5.0/plugin_ref_plugman.md.html#Using%20Plu
gman%20to%20Manage%20Plugins