mixpanel pushnotification support.

Android app build with phonegap-plugin-push is not optimized to receive the default, web-app generated Mixpanel pushes.
For the notification to appear on Android devices, currently one needs to include a custom payload in Mixpanels "Custom Data":
{
"title":"Title",
"body":"Content"
}

This is an error prone step for any non-technical using Mixpanel to send notifications.

Related Issue

Motivation and Context

To Receive notifications from mixpanel with out the need for using the json format like above, i need to modify two files in the plugin.

How Has This Been Tested?

Tested Android app by pushing notifications from mixpanel with out using custom JSON format.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

@yaswanthsvist thanks for the PR