Added export declaration support.
See http://people.mozilla.org/~jorendorff/es6-draft.html#sec-exports, ariya/esprima#216, square/es6-module-transpiler#63 for latest decisions on `export default 42` vs `export default = 42` (first wins, so returning AST as it was in older Esprima Harmony and used by https://github.com/square/es6-module-transpiler/).