Extension module runs with Python 3

[dzhoshkun]

The C extension module has been refactored with C macros that enable conditional compilation based on the Python version (closes dzhoshkun/ndicapi#11). The changes have been tested with both Python versions on:

• Ubuntu 16.04.3 LTS 64-bit (Python 2.7.12 and 3.5.2)
• macOS Sierra 10.12.6 (Python 2.7.14 and 3.6.3)

(For the details of the testing procedure, please see dzhoshkun/ndicapi#14)

Other notable changes this PR introduces:

• The building and installation of the Python extension module has been decoupled from the building of the C++ library (closes dzhoshkun/ndicapi#12).
  • The CMake configuration now merely generates the setup.py file.
  • Also the installation instructions now only include a pip install. This is a cleaner way to install the extension module, as it takes care of removing all components (including the installed dynamic library file, i.e. ndicapy.so on Linux) when pip install ndicapi is called.
• The CMake build option BUILD_SHARED_LIBS is now ON by default (the Python extension module requires a shared library).
• The Python extension module has been renamed from pyndicapi to ndicapy as the latter is shorter and more Pythonic. Closes dzhoshkun/ndicapi#13

The following guidelines / documentation links are useful for porting Python 2 extension modules to Python 3:

• https://docs.python.org/2.7/c-api/typeobj.html
• https://docs.python.org/3/c-api/typeobj.html
• https://docs.python.org/3/howto/cporting.html
• http://python3porting.com/cextensions.html
• http://py3c.readthedocs.io/en/latest/ext-types.html

[dzhoshkun]

It's beyond me why GitHub shows 76 different commits here, while the to-be-merged commits are much less than that. I've merged my issue branch onto my own master branch. So will close this PR and submit a new one directly from my own master branch. That might resolve this problem.