Adds precision capability to dateTimes, fixes #20

Specifically, adds the corresponding types from CybOX
(DateTimeWithPrecisionType and DateTimePrecisionEnum) and changes
xs:dateTime elements to DateTimeWithPrecisionType while adding
timestamp_precision attributes to elements with attributes of type
xs:dateTime
Added annotations stating the timestamps should be included if known

closes #21
Merge pull request #101 from STIXProject/issues_20_21

Timezone and time precision