[doc] Add link to Xephon-K and close all issues

- Fix #15, Fix #14, Fix #13, Fix #12, Fix #11, Fix #10, Fix #9
- Fix #6, Fix #4, Fix #3, Fix #2
- Fix #1

Support type checking and String

- recovery is taken for symbol exceptions
- type checking is taken, though ANY_TYPE is not properly handled yet
- most of #4 is finished

First working interpreter

- only integer is supported
- negative integer is not supported, may need to modify ANTLR grammar
- symbol table works, but lack the following
  - error handling
  - error recovery
- type check is not considered (but should and will be) when building the AST, and evaluation
just use cast and assume everything is right
- used a bunch of instanceof, I think it can be avoid even using the
visitor pattern because I saw a bunch of them in ANTLR generated parse
tree code