Parse import __future__ from x as a future import statement

[rewinfrey]

According to the Python docs, future statements are a special form of import statements.

Currently parsing the following code

from __future__ import print_function

produces

(module [0, 0] - [1, 0]
  (import_from_statement [0, 0] - [0, 37]
    (dotted_name [0, 5] - [0, 15]
      (identifier [0, 5] - [0, 15]))
    (dotted_name [0, 23] - [0, 37]
      (identifier [0, 23] - [0, 37]))))

but ideally this would emit a tree like:

(module [0, 0] - [1, 0]
  (future_import_statement [0, 0] - [0, 37]
    (dotted_name [0, 23] - [0, 37]
      (identifier [0, 23] - [0, 37]))))