Documented that element content should be empty when using an idref
Added External_ID to Incident
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
Merge branch 'version_1.1' into issue_53