move PBKDF2 to lib

--HG--
rename : pbkdf2.js => lib/pbkdf2.js
move PLAIN to mechanisms subdir

--HG--
rename : plain.js => mechanisms/plain.js
move SCRAM-SHA1 to mechanisms subdir

--HG--
rename : scram-sha1.js => mechanisms/scram-sha1.js
Grunt reflecting reality
factory to 100% coverage
expand docs for factory