KWSys: Add hash function for std::string

Added hashing fuction for std::string.  This adds default support for
std::strings to KWSys hashing containers.

Author: Bradley Lowekamp <blowekamp@mail.nih.gov>
Suggested-by: Arnaud Gelas <arnaud_gelas@hms.harvard.edu>
Change-Id: I7e7a0c356b73d19868a3df1db57b702ec7fffe9d