Add cmake build system and VS2013 support

I needed to build project based on protobuf-c (osm2pgsql) with Microsoft Compiler under Windows. Earlier the library already had MSVC support, so patching was minimal (two changes in protobuf-c.c ).

However, autotools build system is very hard to use on Windows (only with cygwin/mingw with many incompatibilities). So the CMake build scripts were created. The scripts are also usable on Unix systems (tested on FreeBSD 10). To build user needs to run something like
```
cmake .. -G "NMake Makefiles" -DCMAKE_INSTALL_PREFIX=c:\libs
nmake install
```
(protobuf installed in ``c:\libs``  is needed). Tested on free Visual Studio Express 2013.

 I tried to include testing support (`ctest` will run all the tests, alternative to `make check`) but did not bother to use valgrind or gcov (I doubt cmake will replace autotools completely for now).