based on: https://stackoverflow.com/questions/2391194/what-is-gs-pthread-equiv-in-clang

still need to look at windows case

…ild/nuget

* fixed bug of method PyString_FromString

* fixup! fixed bug of method PyString_FromString

* removed spaces

* Update appveyor.yml

* Update requirements.txt

* Update conversiontest.cs

* Update test_conversion.py

Travis now has msbuild 15 available, which breaks the no-arg build since it's using the pythonnet.sln which is msbuild 14. Fixed by forcing the use of msbuild 14.

Appveyor on py34 is failing to build psutil; newer python and py27 are using binary releases so the problem doesn't show up. I couldn't figure out how to fix it, so I made it an allowed failure.

As requested here: pythonnet#538 (comment)

The Unity editor unloads the C# domain and creates a new domain every time it recompiles C# sources.

This caused two crashes.
1. After a domain unload, python is now pointing to a bunch of objects whose C# side is now garbage. Solution: Shutdown() the engine on domain reload, which calls Py_Finalize.

2. After a domain unload, Py_Finalize, and a new Py_Intialize, python still keeps pointers in gc for any python objects that were leaked. And pythonnet leaks. This means that python's gc will be calling tp_traverse, tp_clear, and tp_is_gc on types that are implemented in C#. This crashes because the implementation was freed. Solution: implement those calls in code that is *not* released on domain unload. Side effect: we leak a page on every domain unload. I suspect but didn't test that python gc performance will be slightly faster.

Changes required to implement and test:
3. Use python's platform package to determine what we're running on, so we can use the right mmap/mprotect or VirtualAlloc/VirtualProtect routines, the right flags for mmap, and (theoretically) the right native code. Side effect: to port to a new platform requires updating some additional code now.

4. New unit tests. (a) A new test for domain reload. It doesn't run under .NET Core because you can't create and unload a domain in .NET Core. (b) New unit tests to help us find where to add support for new platforms.

Minimal Py_IntializeEx change

Convertes System.Decimal to decimal.decimal and vice-versa

Converts System.DateTime and System.TimeSpan to datetime.datetime and datetime.timedelta and vice-versa

DECREF'ing datetime timezone argument when DateTimeKind is Unspecified was causing `Fatal Python error: deallocating None` because the object was set to `Runtime.PyNone`.

Fixed the input to datetime constructor as we were passing milliseconds, where it should be microseconds.

- C# decimal conversion will use C# double and python float due to the
big performance impact of converting C# decimal to python decimal.