Implement function definition and re-definition in delta execution
Remove subtree cache for function re-definition
Merge pull request #140 from ke-yu/StableBranchWithTestcases

Add StartUp() and ShutDown() methods to HostFactory
Merge pull request #139 from junmendoza/FunctionDeclarationInCodeBlocks

Function declaration in code blocks
Add method to AstFactory; init ReturnType for FunctionDefinitionNode
Merge remote-tracking branch 'upstream/StableBranchWithTestcases' into StableBranchWithTestcases