build: package only necessary content

[ricellis]

Checklist

• Added tests for code changes or test/build only changes - build only
• Updated the change log file (CHANGES.md) or test/build only changes - build only
• Completed the PR template below:

Description

Package only necessary content.

Fixes part of i303

Approach

Add files property to package.json

Schema & API Changes

• "No change"

Security and Privacy

• "No change"

Testing

• N/A build or packaging only changes, ran npm publish --dry-run

npm notice 
npm notice 📦  @cloudant/[email protected]
npm notice === Tarball Contents === 
npm notice 11.3kB LICENSE                      
npm notice 16.2kB README.md                    
npm notice 16.5kB app.js                       
npm notice 2.7kB  bin/couchbackup.bin.js       
npm notice 2.1kB  bin/couchrestore.bin.js      
npm notice 9.3kB  includes/backup.js           
npm notice 1.3kB  includes/change.js           
npm notice 1.8kB  includes/cliutils.js         
npm notice 3.4kB  includes/config.js           
npm notice 3.0kB  includes/error.js            
npm notice 1.4kB  includes/liner.js            
npm notice 2.1kB  includes/logfilegetbatches.js
npm notice 2.6kB  includes/logfilesummary.js   
npm notice 5.4kB  includes/parser.js           
npm notice 7.0kB  includes/request.js          
npm notice 1.2kB  includes/restore.js          
npm notice 2.7kB  includes/shallowbackup.js    
npm notice 4.0kB  includes/spoolchanges.js     
npm notice 5.9kB  includes/writer.js           
npm notice 1.6kB  package.json    

Monitoring and Logging

• "No change"