Added parsing of ignore annotation for import statements.

This implements @JukkaL's suggestion of adding `# mypy: ignore` annotations in #500.

In this PR, only ignore annotations on import statements are allowed, but I think it would be easily extendable to other statements as well. Every statement `Node` could have an `annotations` field that could be used during semantic analysis or type checking.
Should we also support this for `from m import x` at least? Maybe the imported names such as `x` should be defined and have type `Any`. Or alternatively, we could require all code that refers to them to use `if not MYPY:` but that would be less convenient.