Supports Token Setting Functionality + New Scopes added

1.) Getting and Setting Tokens are easily implemented using the client library without instantiating a new instance :
https://github.com/intuit/oauth-jsclient#getter--setter-for-token

2.) Issues Fixed :
#11
#13

3.) New scopes added :
Payroll: com.intuit.quickbooks.payroll,
TimeTracking: com.intuit.quickbooks.payroll.timetracking,
Benefits: com.intuit.quickbooks.payroll.benefits,