Support a transaction method on databases, which returns a Transaction object, following the Database API anand allowing commit. Needs work, buggy, isn't integrated, can't rollback yet. God help me I need unit tests. Also, added a putmany method to put multiple objects in one operation.
Merge branch 'master' into 29-transactions
[#29] In all I/O operations, use the new transaction wrapper. Plasmid now handles multi-operation transactions.