Add selectors module to gunicorn codebase.

[berkerpeksag]

Closes #886.

[benoitc]

Thanks! That will help a lot to solve #792 and #872.

I wonder since the module is only needed on py26 we could make it optional and import it only if it missing. What do you think?

[berkerpeksag]

If I remember correctly, this module is only not needed on Python 3.4+. But, yes we can use it like the following pattern:

try:
    import selectors  # for Python 3.4+
except ImportError:
    import gunicorn.selectors as selectors

[benoitc]

@berkerpeksag yup this wht I meant, though I would also rename _selectors . ANyway +1 for the merge :)

[georgexsh]

@berkerpeksag It would be great if this selector module released as a standalone package.

[benoitc]

LGTM, let's merge it imo. (Quite excited to use it as a base for other features)

About having in another module, I am not sure. We could probably have a gunicorn-selectors module. On the other hand it's really part of our core. IMO that's OK to keep it there.

[berkerpeksag]

Do I need to update NOTICE file before merge?