Add a option `check_unique` in nodes.add #44
Merge branch 'v2.1' into check_unique

Conflicts:
	lib/dataprocessor/nodes.py
Merge branch 'validate_link_bug_fix' into check_unique
Change keyword "check_unique" > "strategy"
Add `check_duplicate`
Add tests
Merge branch 'v2.1' into check_unique
Merge branch 'master' into check_unique
Introduce merge_duplicate()
Fix format of test
Add test code (passed)
Create io.SyncDataHandler and write test #67
Add an option into execute
Add option --data(-d) in dataprocessor
Bug fix and output message
Add document about `merge_duplicate`
Fix docstring format.
Merge pull request #57

Add a option `strategy` to nodes.add.
Moreover, add nodes.check_duplicate and nodes.merge_duplicate.
Merge pull request #82.