use the correct way to convert unicode to string. I also think this warning should be removed since it can happen when unicode_literals from __future__ is enabled.

skip scalar as shape test on pypy. [upstream bug](https://bitbucket.org/pypy/numpy/issue/10/scalar-types-should-not-be-iterable)

fix compile error for binary float function with two different input types, add tests for atan2 atan2pi

The same case is already covered by test_array.py and doesn't have to be covered here again.

fix test_wrapper.test_get_info for old opencl version, adapted from the cffi branch ac7ed3c9

Use the documented struct.calcsize to determine machine word size for better pypy support.