Remove Windows dependency (disable uuid_time_generator by default)

[EvanBalster]

This PR disables uuid_time_generator by default, allowing it to be re-enabled by defining UUID_TIME_GENERATOR.

If neither UUID_TIME_GENERATOR or UUID_SYSTEM_GENERATOR are included, Windows.h will not be included. This avoids some namespace trouble and compile-time overhead.

I was originally planning to add a disable macro rather than an enable macro, but noticed the warning in the source against using uuid_time_generator in production. Nevertheless, this might break compatibility with some existing projects.

[mariusbancila]

Can you please fix the unit tests? With these changes, the build fails:

C:\projects\stduuid\test\test_generators.cpp(260): error C2065: 'uuid_time_generator': undeclared identifier [C:\projects\stduuid\build\test\test_stduuid.vcxproj]
C:\projects\stduuid\test\test_generators.cpp(260): error C2146: syntax error: missing ';' before identifier 'gen' [C:\projects\stduuid\build\test\test_stduuid.vcxproj]
C:\projects\stduuid\test\test_generators.cpp(260): error C2065: 'gen': undeclared identifier [C:\projects\stduuid\build\test\test_stduuid.vcxproj]
C:\projects\stduuid\test\test_generators.cpp(261): error C3861: 'gen': identifier not found [C:\projects\stduuid\build\test\test_stduuid.vcxproj]
C:\projects\stduuid\test\test_generators.cpp(262): error C3861: 'gen': identifier not found [C:\projects\stduuid\build\test\test_stduuid.vcxproj]
C:\projects\stduuid\test\test_generators.cpp(275): error C3861: 'gen': identifier not found [C:\projects\stduuid\build\test\test_stduuid.vcxproj]

[EvanBalster]

Fixed (eliminate the test when UUID_TIME_GENERATOR is not defined).

I forgot to mention: this pull request relates to issue #33.