ES6 Module Syntax

* adds `ImportGlobSpecifier` and `ImportDefaultSpecifier` def
* updates `specifiers` field from `ImportDeclaration`
* removes `kind` field from `ImportDeclaration`
* removes `default` field from `ExportDeclaration`

_WIP_ this is bound to https://github.com/ariya/esprima/pull/287 and https://github.com/benjamn/recast/issues/112