implement timestamp and interval syntax (Fixes #442)
- `date` and `time` also parsed, but not handled in planner
- all these operators now parsed with higher precedence (same as unary -, +, and distinct)
- both accept the syntax accepted by threeten
- intervals represented as milliseconds for mongodb
add timestamp, date, time, and interval to the Arbitrary for Node
Merge pull request #444 from mossprescott/ready/442

implement timestamp and interval syntax (Fixes #442)