AST: Rename `char` properties to `symbol`

`char` is a reserved word in ES3 so any scripts consuming the AST that need to support ES3 environments should always use bracket notation rather than dot notation to access any properties by that name. http://mothereff.in/js-properties#char Renaming any `char` properties to `character` or (even better IMHO) `symbol` and using that name consistently throughout the AST avoids this issue.
Merge pull request #22 from mathiasbynens/char-to-symbol

AST: Rename `char` properties to `symbol`