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ARROW-477: [Java] Add support for second/microsecond/nanosecond timestamps in-memory and in IPC/JSON layer

Changes include:
- add support for TimeStamp data type with second/microsecond/nanosecond time units
- add an additional readLong() method to timestamp readers to support reading raw long values
- add a simple test case for timestamp readers and writers

Author: Jingyuan Wang <jingyuan@live.com>

Closes #303 from alphalfalfa/arrow-477 and squashes the following commits:

0199574 [Jingyuan Wang] rename TimeStamp to TimeStampMilli
068e47f [Jingyuan Wang] use a test value that exhibits micro/nanosecond truncation when converting timestamps to JODA DateTime
bef2330 [Jingyuan Wang] fix a typo
9b4d7b4 [Jingyuan Wang] add support for timestamp data type with second/microsecond/nanosecond time units