4 new functions: getUserBalance, calculateReqCost, checkCredit, insertOrUpdateTransaction

Four new functions

getUserBalance() - fetches balance for the user who is logged in and
returns as double

calculateReqCost(imageID,startTime,endTime) - checks if the logged in
user has sufficient credit to reserve the given resource for given time
- returns true if yes else false

inserOrUpdateTransaction(requestID) - does two things: if there is no
entry for transaction in vcenttrxn, it creates one, deducts the
reservation cost from user balance and returns the balance; if there is
already an entry in the vcenttrxn table then it updates the entry for
the new cost calculation and returns only the difference in cost that
can be used by notification module to send an email to user - assuming
that there is only extension possible

calculateReqCost(imageID,startTime,endTime)