ES6 Module Syntax

* adds `ImportNamespaceSpecifier` and `ImportDefaultSpecifier` def
* updates `specifiers` field from `ImportDeclaration`
* removes `kind` field from `ImportDeclaration`
* removes `default` field from `ExportDeclaration`

Gathering early feedback about the changes needed for the final syntax.

_WIP_ this is bound to https://github.com/ariya/esprima/pull/287 and https://github.com/benjamn/recast/issues/112
As I [commented](https://github.com/ariya/esprima/pull/287#discussion_r18295065) over on the Esprima PR, I think we need to keep the `.default` field.