New Windows Shell API mapping: SHGetSpecialFolderPath

Timothy and Daniel,

as discussed https://groups.google.com/forum/?fromgroups#!topic/jna-users/KUpYp8rzFwk before, here is the first of a few small extensions.

This one maps the Windows Shell API function "SHGetSpecialFolderPath" and provides a unit test for it. As we are porting some software 1:1 from C++ to Java, we needed to use this. I know that the same can be done with the already existing GetFolderPath, but we (and possible more) people like this (rather deprecated) function more, as it allows to be as simple as GetFolderPath(CSIDL), but has the option to create paths.

Would be great if you would accept this contribution.

More APIs are one the way. :-)