Add native Visual Studio 2015-2017 build support

Also add missing getopt.h header for gostsum and gost12sum tools build in MSVS (borrowed from https://github.com/skandhurkat/Getopt-for-Visual-Studio).

Add install via CMake.

New INSTALL.md file Second attempt

New INSTALL.md file instead of old CMake_ReadMe.md; Also moved "CONFIGURATION FILE" section from README.gost into INSTALL.md. That section were reformatted to MarkDown, but it's content were not changed

Minor README.gost fix (I have shown my abstract to proofreader to fix grave mistakes).

README.gost: Abstract rewritten, outdated text at the end of file is removed                        

necessary to use the GOST engine.

Specify that project uses only C language, to avoid complaints of CMake if no C++ compiler exist on the system

Add support of legacy format for GOST private key to make BouncyCastle happy.

Address compiler warnings and build with CMAKE_C_FLAGS

like the README suggests.

Fix error in printf() invocation

where they reside