Code review changes for the StatusBar plugin

From Andrew Grieve in the ML:
http://callback-dev.markmail.org/thread/y345lht3ggjmyj5q

Looking at the Keyboard & Statusbar plugins, would you be opposed to using:

<clobbers target="cordova.plugins.statusBar" />

instead of:

<clobbers target="window.StatusBar" />

Would require a major version bump, but probably it has little usage since
it's new & in labs still?

Thinking here is that since it's not implementing any spec, we should keep
symbols off of window / navigator objects. Would be good to encourage
plugins not to pollute the global namespace I think.

Other points from code-review standpoint:

• Why make StatusBar a function? It has no prototype methods.
• You're calling exec() from the module scope. You should add a <runs/> to
  the <js-module> to ensure it gets run at start-up. You should also delay
  deviceready until it executes. cordova-plugin-device has an example of
  this.
• Might be nicer to make the status-bar state a callback with
  keepCallback=true so that the native code is agnostic to where the
  namespace is mapped.