Test update

Updated tests for new desired behavior.  All usages of -CertificateThumbprint parameters have been replaced with just -Certificate, and there's a new test to make sure that file system paths or cert:\ paths (including relative paths) can also be used.
Updated Protect-Data -Certificate parameter
Updated -Certificate parameter of Unprotect-Data
Code updates complete; tests passing

Still need to add deprecated warnings for CertificateThumbprint parameter, and update documentation