Rename Transaction to Tx.

I changed the Transaction/RWTransaction types to Tx/RWTx, respectively. This makes the naming
more consistent with other packages such as database/sql. The txnid is changed to txid as well.
Consolidate Tx and RWTx.
Merge pull request #60 from benbjohnson/tx

Rename Transaction to Tx.
Merge pull request #61 from benbjohnson/merge-tx

Consolidate Tx and RWTx