Fix inconsistent tx state with database/sql.

[sqweek]

The semantics of sql.Tx.Commit impose that the transaction is
finished and cleaned up by the time the driver's Commit function
returns. However sqlite3 leaves the transaction open if COMMIT
fails due to an SQLITE_BUSY error, so we must clean it up.

Closes #184.

[coveralls]

Coverage decreased (-0.4%) to 66.38% when pulling 727ad20 on sqweek:issue184 into 22d7351 on mattn:master.

[sqweek]

To test the change I used this little program: https://play.golang.org/p/wnMn2OVhfn

1. Run the program a few times to create the database (/tmp/issue184.db) and insert some rows
2. In a separate terminal run the commands:
   1. sqlite3 /tmp/issue184.db
   2. sqlite> BEGIN;
   3. sqlite> SELECT * FROM tab;
3. The sqlite3 process has a read-lock on the database. Run the program again.
4. The program will get a database is locked error trying to COMMIT, as expected.
5. The program sleeps for 5 seconds and tries to tx.Commit() again, which fails because database/sql considers the transaction finished
6. The program tries to tx.Rollback() which fails for the same reason
7. The program calls db.Begin() to open a new transaction, which succeeds

Before the change, the db.Begin() call failed with error cannot start a transaction within a transaction.

I'm not sure how easy it would be to isolate this into a test case.

[mattn]

Thanks.