Pyprimesieve

------------------

- Python 3 support
- No longer assume `g++` in `setup.py` but rather tries installing with OpenMP and retries if that fails

------------------

- added to pypi

------------------

- Parallel prime summation is now a class
- Threads calling callback method are NOT synchronized so faster on large input
- Updated README in bench to reflect new timings (small input is slightly worse but large is twice as fast)

------------------

- Fixed compilation warnings
- extern "C"
- Added more information regarding installation to README; updated docstring

------------------

- primes_nth uses callback class instead of creating any PyObjects in order to call primes
- Caused a memory leak and was unnecessary

------------------

- Distutils setup.py: got multithreaded summation compiling with OpenMP
- Removed all code to manually compile executable
- CC and CXX environment variables set to avoid clang (and other non-openmp compilers) use
- Replaced executable code with parallelized function in the actual extension
- Rewrote primes_sum and primes_nth Python functions in C++ and removed Python script completely