Ensure that options are specified only once in VS201x projects.

If the value of an option generated by default by Bakefile was also specified
explicitly by the user by setting vs201x.option.xxx property, its value
appeared twice in the generated project and the user-specified value was
actually overridden by the default one.

Avoid this by ensuring that we take the user-specified properties into account
last and replace any previous values for these properties with their values
instead of simply adding them.

Closes #13.
Merge pull request #14 from vslavik/vs201x-unique-props

Ensure that options are specified only once in VS201x projects.