Add a REST call for adding an Expense to a Particular User

[vignesh-6145]

Requirements

• [x] A REST API call should add expense under a particular user
• [x] Should return meaningful response codes
• [x] Should return the URI of the added Expense in the headers
• [x] An Individual Controller for Expenses should work without user information

[vignesh-6145]

Detailed Update

• A POST call implemented which was able to insert the passed expense
• Upon successful insertion it returns the URI of the particular Expense
• Separate Controller for Expense operates solely on Expense information
• While retrieving the Expense it also consists links for Attached user, that particular Expense, all the expenses of the particular user