Add Plus event (#517)

- Plus event interface
- SimplePlus implementation and factories
- Tests for the factories
Add EventBuilder abstraction
Add PlusBuilder and a test class (#517)
Add plus count summarizer (#517)
Generalized parsing of events from text files

- TextEventDAO parses events from text line-by-line
- EventFormat describes how to parse an event from a line
- Use this to parse ratings
Simplify and reuse code for rating parse contexts
Move user and item DAO tests to .text
Use field lists to parse ratings
Add event type definition
Refactor rating format into column format
Directly use list of fields
Rename EventBuilder.clear
Use and reset context in text event DAO
Don't use type parameters on the event formats
Support arbitrary event types on the command line
Add event types for parsing Plus data