Add microsecond precision

[cverges]

This is a feature request for adding microsecond precision to joda-time.

The need for microsecond precision is more common than it used to once be. For example, support for this level of precision has recently been added to MySQL (5.6.4+ via DATETIME(6)) and has existed in other relational databases (Oracle, PostgreSQL, etc.) for some time.

[jodastephen]

This is pretty near to impossible in Joda-Time as millis is baked in everywhere. Fortunately, java.time (JSR-310) in Java SE 8 supports nanosecond precision.