Attempt to implement replacement for using select.poll on macosx

[fizyk]

With using select.select

[mmaslowskicc]

Missing comparison?

[mmaslowskicc]

Can we reliably detect if poll is available? (I remember experiencing some issues with maybe gevent hiding it.)

[fizyk]

@mmaslowskicc It appears to be dependand of the compiler used for macos, if it's gcc, then the poll is available, and the simplest test would be to check for the poll function in select module. (we won't be able to test that on travis though - hence I'm looking for the platform name instead)

[coveralls]

Coverage decreased (-1.08%) to 82.878% when pulling 98118b8 on fizyk:issue-331 into 4a503ce on ClearcodeHQ:master.

[mmaslowskicc]

Or we could support choosing any implementation at runtime, and tests would check both. Your approach is ok too, it might fix the problem and keeps the code tested.