Refactor MSVC project files

Ran into some trouble building the library using MSVC 2013. This patch fixes the following issues:

 - Header include paths were incorrect
 - verify_poly1305_53.c was being included, which resulted in DLL compilation failure.
 - Settings across platforms and Debug/Release builds were inconsistent (e.g., some had no prebuild step).

Additionally, moved global project settings (preprocessor directives, prebuild step, include paths) into a separate properties file, which gets included by every project.