Bug 12487: SQLHelper replacement - C4::Contract

Bug 12487: SQLHelper replacement - C4::Contract

With this patch, the subroutines GetContract, GetContracts, AddContract, ModContract and DelContract uses DBIx::Class instead of C4::SQLHelper

Test plan:
1) Apply the patch
2) Execute the unit tests by launching:
prove t/db_dependent/Contract.t
3) The command has to be a success :
t/db_dependent/Contract.t .. ok
All tests successful.
Files=1, Tests=43,  2 wallclock secs ( 0.04 usr  0.01 sys +  1.49 cusr  0.05 csys =  1.59 CPU)
Result: PASS

4) Log on with a superlibrarian permission
5) Go on the page acqui/supplier.pl (Acquisitions > Button "New vendor")
6) Record a vendor with a nonzero "name"
7) Go on the page admin/aqcontract.pl (click on the "Contracts" item in the menu)
8) Click on the button "New" > "Contract" and record a new one
9) Verify the displayed data are correct about the contract
10) "Edit" the contract with different values and verify the data are updated
11) Click on "Delete" in order to delete the contract, verify the displayed data are correct but cancel the operation
12) Click on "New" > "Basket" and verify there is the created contract in field "Contract", then record a basket by selectioning the created contract
13) Verify the contract name displayed is correct
14) Record an active budget and a fund linked to this budget
15) Go on the new basket (Home > Acquisitions > Search the created vendor)
16) Click on "Add to basket" then "From a new (empty) record" and verify the displayed contract name is correct, then cancel
17) Click on "Delete this basket"
18) Click on "Contracts" (in the left menu) in order to go on aqcontract.pl, then "Delete" the created contract
19) The contract is not displayed anymore in the vendor page

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Work as described following test plan.
Tested on top of Bug 12493
Tests pass
No koha-qa errors

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>