Update units.js

Added pattern 'Xft Yin' and 'pounds'
Update units.js

Changed comment style since double-slashes would end code evaluation on a single line bookmarklet.
Update units.js

Forgot a comment
Update README.md

Bookmarklet instructions
Update units.js

Updated pattern for 'lbs'
Update safari extension
Merge pull request #2 from mirosval/pr/1

Merge ability to create bookmarklets