Extract variable tracking logic to VariableInspector module
Add class documentation of VariableInspector

This closes #208.
Add specs of VariableInspector and fix NodeScanner.scan_nodes_in_scope
Add new cop ShadowingOuterLocalVariable (#207)
Fix a variable shadowing
Merge pull request #299 from yujinakayama/shadowing-outer-local-variable

Add new cop ShadowingOuterLocalVariable