Allows for the capture of the precision of dates and times

Addresses #34, but does not address the use of precision on the date/time-based CybOX ObjectAttribute types so does not fix it
Addresses #11 through documentation and the addition of the precision attribute

Specifically, each of TimeObjectPropertyType, DateObjectPropertyType, and DateTimeObjectPropertyType had the following modifications applied:

* Documentation explaining in more detail expected behavior for pattern comparisons
* The addition of the precision attribute. This was accomplished by breaking the restriction out into a separate abstract type, and extending that type with the extra attribute. This is necessary because in XML schema the same complexType block cannot contain both a restriction and an enumeration

In conjunction with commit fa7d54, this fixes #34
Added documentation stating a timezone is suggested to all date/time fields

Fixes #35.
Updated annotations to better specify timezones and precisions

* Specified using timezones only if they are known
* Added annotations clarifying that digits of a time/date beyond the given precision should be zeroed-out.
Merged changes from the 2.1 branch
Merge pull request #213 from CybOXProject/issues_11_34_35

Issues 11, 34, 35 (Date and Time fixes)