Update version and SDK - 3.1.8.0
Fix link in README.
Fix a memory leak in credential initialize.  Do not dup pre-set value if field is dynamic (and is going to then also dup a value)
nobug here - but less ptr indirection for more readable code
Fix issue #199 - Set field string to NULL after clearing it.  Switching away from tile in win8 causes us to clear our fields as we go out of scope, but logonui also sets an "" value (this seems to be new behavior).  If the clear happens before the set, the set would bomb because it tries to double free.