Add MapScanCAS() to capture a prev row safety.
Remove an unnecessary if branch.
Refactor Scan/ScanCAS/MapScanCAS common logics
Add comment the motivation of MapScanCAS()
Merge pull request #250 from matope/MapScanCAS

Add MapScanCAS() to capture a prev row safety.
Merge branch 'master' of https://github.com/gocql/gocql

Conflicts:
	AUTHORS