[WIP] Add AppVeyor CI setup on Windows

[rth]

This PR aims to track progress on building simhash-py on Windows using the Microsoft Visual Studio compilers, as discussed in issue #29. It adds,

• a appveyor.yml setup script (that would require to activate Appveyor CI on this repository)
• makes paths in setup.py OS independent

This is still work in progress, currently Windows builds for all Python versions fail as discussed in the parent issue #29, in particular detailed error messages can be found for,

• PY2.7 -> build fails
• PY3.4-> build fails
• PY3.5 -> build fails

Notes

• this SO post seems relevant, msinttypes could be a solution.

[rth]

Support of multiple Python versions on Windows may be too challenging a task given the use of simhash-cpp of the C++11 standard, and the poor support of this standard by earlier versions of MSVC compilers.

Therefore restricting this PR to Python 3.5 (or even 3.6) that use the latest (and most complete) MSVC 2015 compiler might a more reasonable first step.

Update: latest Appveyor CI output can be found at https://ci.appveyor.com/project/rth/simhash-py