replaced __import__ by importlib.import_module.

Since python 2.7, importlib.import_module is the recommended way to
import modules programmatically.

From __import__'s doc:

    Import a module. Because this function is meant for use by the
Python
    interpreter and not for general use it is better to use
    importlib.import_module() to programmatically import a module.
Replaced remaning __import__(module) calls.

This commit replaces the statements __import__(module) as the previous
replaced the statements __import__(module, {}, {}, ['']).

At first I thought leaving the single-argument calls, but perhaps it's
better to be strict rather than having exceptions to the rule in this
case.
Merge branch 'master' into import-module
Merge pull request #445 from darkrho/import-module

Use `importlib.import_module` instead of `__import__`