COMP: remove exporting String as VS10 attempts to export its parents (basic_string) which causes all kinds of multiply defined symbols at link time.

This converts the KWSys license to a pure 3-clause OSI-approved BSD
License.  We drop the previous license clause requiring modified
versions to be plainly marked.  We also update the KWSys copyright to
cover the full development time range.

STYLE: Remove references to std:: inside KWSys, even in comments.  This will allow a commit check to be added.

ENH: Added kwsys::String class to shorten debugging symbols and error messages involving std::string.