Report transaction conflicts, and tentative account balance fix

This adds a "walletconflicts" array to wallet transaction JSON output, identifying other wallet transactions that also spend one of the inputs of this transaction.

Example output from the txnmall.sh regression test, spending 1 BTC from account "foo", then spending 2 BTC of the change from that transaction and spending it from account "bar".  After a mutant of the "foo" transaction is then mined listtransactions reports:

```
    {
        "account" : "foo",
        "address" : "n2rhjnG8Gg24Dcx66u4KrtiUfxPhPw7suV",
        "category" : "conflicted",
        "amount" : -1.00000000,
        "fee" : 0.00000000,
        "confirmations" : -1,
        "txid" : "650e1c572593ba03e407faebb613c1d449dfc450d3d3f3868566367f482a105b",
        "walletconflicts" : [
            "587573bb1b620604eb39f75377358411f34dd133a8846d50e8947091b208eb1b"
        ],
        "time" : 1392405954,
        "timereceived" : 1392405954
    },
    {
        "account" : "bar",
        "address" : "n2rhjnG8Gg24Dcx66u4KrtiUfxPhPw7suV",
        "category" : "conflicted",
        "amount" : -2.00000000,
        "fee" : 0.00000000,
        "confirmations" : -1,
        "txid" : "222b3d25454a38de6ecd1c493692c7fce8848a2f8586825e4b32fe63c1f41d43",
        "walletconflicts" : [
        ],
        "time" : 1392405954,
        "timereceived" : 1392405954
    },
    {
        "account" : "foo",
        "address" : "n2rhjnG8Gg24Dcx66u4KrtiUfxPhPw7suV",
        "category" : "send",
        "amount" : -1.00000000,
        "fee" : 0.00000000,
        "confirmations" : 1,
        "blockhash" : "0000996962dbab0bf203ab187a683e7c610b2e946b3027c7ef167977dc1ea317",
        "blockindex" : 1,
        "blocktime" : 1392405954,
        "txid" : "587573bb1b620604eb39f75377358411f34dd133a8846d50e8947091b208eb1b",
        "walletconflicts" : [
            "650e1c572593ba03e407faebb613c1d449dfc450d3d3f3868566367f482a105b"
        ],
        "time" : 1392405954,
        "timereceived" : 1392405954
    }
```