Refactored Purchase Request Class.

Purchase Request Class now has two main functions: 1. create 2. retrieveexisting purchase requests
Change is only made to how we retrieve the purchase request.
Combined all the purchase request list to one list (@@list). Before there was one for pending, hold, and completed for a total of three.
Instead, separated retrieve purchase request into three separate methods
1. retrieve all 2.retrieve by request number 3. retrieve by status
Added a find_status function in the purchase_request_repo which has not been tested officially yet.
New Purchase Request Class has passed all tests in purchase_request_spec.rb