add_shell: get __builtins__ directly instead of from __main__

[quarl]

In some situations, main.builtins doesn't exist at the time
add_shell() is called. The old code assumed that it does, and would segfault.

Before this commit:

$ python -c 'import IPython; \
             app = IPython.terminal.ipapp.TerminalIPythonApp.instance(); \
             app.user_ns = {}; app.initialize(); app.start()'
In [1]: import apsw
<segmentation fault>

This was first discovered when using pyflyby:

$ py
In [1]: import apsw
<segmentation fault>

To avoid that problem, this commit changes the code to import builtin as a
module directly.

[rogerbinns]

This patch exposes a different problem. In some situations, the builtins module doesn't have import yet. You can see if you run tools/megatest.py which compiles fresh python versions and runs the test suite. The symptom is this when running the tests:

Traceback (most recent call last):
  File "<string>", line 2, in <module>
ImportError: __import__ not found

Thank you for finding this issue and I would like to fix it, but it is going to take more work from me first.

[rogerbinns]

The good news is that the shell is now broken out from the extension (#356) so this is no longer an issue. apsw will import just fine,