Cleanup

- Deprecate 'save' event
- Add TransactionQueue.getMinLedger(), use this as ledger_index_min
  in account_tx request on reconnect
- tx.sign() no longer accepts a callback
- Add various setters and jsdoc to transaction.js
- Normalize setters, e.g. sourceTag() and destinationTag()
- Minor optimization in call to tx.hash() in TransactionManager prior
  to submit; allow `serialized` argument to tx.hash() such that the
  transaction is not serialized twice
Make LastLedgerSequence offset configurable, default to 3
Do not bump LastLedgerSequence on resubmit
Rework tef and tel-class transaction error handling

Do not finalize tef or tel-class errors until LastLedgerSequence is
exceeded. Transactions that fail in this way will now be aborted
with a tej-class error. Errors like tefALREADY and tefMAX_LEDGER
should now be opaque to the user and have no consequence in
determining a final state.
Merge pull request #221 from ripple/transaction-updates

Transaction updates
[FEATURE] improve memo support

- add MemoFormat property for memo
- MemoFormat and MemoType must be valid ASCII
- Memo content is converted on the serialization level
- add parsed_* version of Memo content if the parser understand the format
- support `text` and `json` MemoFormat

[FIX] double serialization overriding Memo contents

The copy made in from_json wasn't a deep copy