Remove ENUM from keyword tokens list.

It's a future reserved word, not a full-blown keyword.
Allow use of keywords in property names.

According to ECMAScript 5.1, this is allowed:

    x = {var: 10};
    x.var = 7;

So I introduced a new rule IdentName that encompasses identifiers,
keywords and reserved words, which are all allowed to be used as
property names.
Bump version to 0.0.4.