POST /templates, with excel template file as payload.
    This saves the template and returns a unique token for that template in the response. Henceforth this token can be used for referring this template.
PUT /templates/<template-token> with excel template file as payload. This updates the existing the template with the new template that is sent as the payload and returns the template token 
GET /templates/<template-token> This fetches the corresponding template for the given excel template token
POST /xls/<template_token> with JSON data as payload.
    This will use the template corresponding to the token in the URL and generate an Excel file using the template and JSON data. The URL using which this excel file can be downloaded is returned in the response.
GET /xls/<generated_excel_token>
    This returns the excel file for given token.
PUT /xls/<template_token> with JSON data as payload.
    This will update the template corresponding to the token in the URL. The URL using which this excel file can be downloaded is returned in the response.