Add check number and payment instrument to financial transaction table

1. Add two columns to civicrm_financial_trxn in schema:

• check_number VARCHAR 255
• payment_instrument_id INT (implicit FK to payment_instrument option group values)

2. Modify 4.3 alpha upgrade to add these columns.

3. Modify contribution form postProcess function to write these values to financial_trxn record (in addition to contribution record) if they are set.