Improve Checker#checkDirectory method

Make it aware of "fileExtensions" option

Ref #445
Place fileExtensions fixtures on right path
Fix recursive descend

* Break down "_shouldProcess" method to two private methods

* Simplify checker module, it contradicts original code idea
but it easier to read

* Add tests for checker module

Fixes #445
Closes gh-447
Improve requireSpaceBeforeBinaryOperators rule

Catch nodes with "VariableDeclarator" type
Improve disallowSpaceBeforeBinaryOperators rule

Catch nodes with "VariableDeclarator" type
Improve disallowSpaceAfterBinaryOperators rule

Catch nodes with "VariableDeclarator" type
Improve requireSpaceAfterBinaryOperators rule

Catch nodes with "VariableDeclarator" type
Prepare for version 1.5.2
1.5.2