Clean up widgets/0002; #349

First this fixes a bug where XHR wasn't handled properly; apparently
it's xhr.onreadystatechange, not .readystatechange. Then we switch to
data-gittip-{base,username} for the data attributes, to make it clearer
for the user. Lastly roll this out as the new code for Embed #2 on the
profile page.
Merge branch 'master' of github.com:zetaweb/www.gittip.com