JSON parsing

1. Modify file and directory structure a bit. Move script related files
   into their own subfolder `script`.
2. Adds an abstract `Parser` class which can be used to build various
   parsers. Existing `Parser` class is now renamed as `ScriptParser`.
3. Adds JSON parser which is capable of parsing JSON into `Value`s. Add
   API methods to `String` and `Request` classes for JSON parsing.
4. Fix an issue where query string is not processed in builtin HTTPD.