Move dependencies on ognl and tools.jar to testCompile
Add package-info.java for ast and ast_junit packages
Define TestStatementVisitor
Remove Stringifiable interface
Introduce ExpressionStrategy interface (empty for now)

An instance of EmptyStrategy is passed as an argument for toString.
Decouple OGNL-dependent code generation from JUnit AST classes
Remove unused BindingTest
Clean up codes
Fix: do not import `ognl.Ognl` if there are no bindings

TestMethodTest.testImportedNames() reported that.
Remove usage of jMockit
Merge refactor-ast-junit-to-java