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
Adds xs:unique constraints, closes #22
Added data markings to uniqueness constraint checks, followup to #22
Correct names for uniqueness constraints, followup to #22
Documented that element content should be empty when using an idref
Added External_ID to Incident
Added fields to VulnerabilityType

Closes #53
Documents the use of KillChainPhaseReferenceType

Specifically, notes the use of the redundant attributes should either be
omitted or, if present, should match the value in the definition.
Addresses #41.
Fixed multiplicity on COA_Requested and COA_Taken

Fixes #14.
Updated: Added External_ID to Incident
Updated:Documented that element content should be empy when using an idref
Named extension schemas more specifically for what they do
Updated:Named extension schemas more specifically for what they do
Merge branch 'issue_17' of https://github.com/JWParlee/schemas-1 into JWParlee-issue_17

Conflicts:
	course_of_action.xsd
	data_marking.xsd
	exploit_target.xsd
	incident.xsd
	indicator.xsd
	stix_common.xsd

Fixes #17
Merge pull request #111 from JWParlee/issue_46

Added External_ID to Incident
Merge pull request #110 from JWParlee/issue_23

Documented that element content should be empty when using an idref
Clarified @id, @phase_id, @kill_chain_id usage

Fixes #40
Merge pull request #115 from STIXProject/issue_41

Document the use of KillChainPhaseReferenceType
Merge pull request #101 from STIXProject/issues_20_21

Timezone and time precision