(feature request): Account reports need the real account opening balance

[jniles]

The account reports have a running balance that always begins at zero. While this gives a nice total of how each line changes over time, we really need to understand the values in context of the total account balance. It will also motivate our users to post from the Posting Journal to the General Ledger to update the account balances.

To be precise:

1. The first line in the report should give the accounts opening balance.
2. The running balance should begin at this balance and continue to the end.
3. The final balance of the account should take into account the opening balance.

[jniles]

To pull this off in a maintainable way, we really need to limit this feature to only posted data, especially with a variable date range.

I'll try implementing this as well as prototype the feature described in #1636. This way it will be ready for the Account Statement module when it is ready.

[jniles]

To prevent collision with #1612, I'll begin implementing #1636 in a separate file.