Support for ImportDeclaration

Fixes #180.

I'm totally new to escodegen, so I'm not entirely sure I'm following correctly the use of your API. I don't think I should be using `generateExpression` or `generateStatement` since import declarations do not take expressions or statements.

There's also an issue in Esprima that makes it parse incorrectly import declarations like `import foo, {bar} from 'foo';` so that should be fixed in the Esprima side.