added BlankLineBetweenDefinitionChecker

This checker detects the class/object/trait/method definition has no blank line above.
[Effective Scala](http://twitter.github.io/effectivescala/index.html#Formatting-Whitespace) suggests "Use blank line between between method, class, and object definitions."

For example.
this method definition will not allowed by checker

    def foo() = ...
    def bar() = ...

to remove warnings we have to write like this,

    def foo() = ...
    
    def bar() = ...