add codegen

- up front work
  - fix num-pi, (10 * Math.PI) -> 10 * Math.PI
  - fix [] -> .at(), .put()
  - fix assign method, a = x -> a_(x)
  - fix Series Literal (1..10), [1,2,3...10] -> 1.series(null, 10)
  - fix BooleanLiteral -> True/FalseLiteral
  - fix adverb as Symnol

- add codegen written in test version
- add tests for codegen
- fix codegen
- clean up
- use esprima
- fix test cases
- add parse demo
- build v0.0.13
Merge pull request #21 from mohayonao/codegen

add codegen