Adding builder methods for new preconditions.

[paulbellamy]

Fixes #508

Note: this is pending on xdr updates to add the new fields to the txn envelope. I've made a guess at their location, but the txn constructor might need updating.

[paulbellamy]

Should setMinTime/setmaxTime, take epoch milliseconds instead of epoch seconds? It would make Date.now(), and new Date().valueOf() type stuff more intuitive in js. But we'd be throwing away some precision. Is there a good example somewhere else we could follow for consistency?