add String() for all setTabValue param 3, workaround for Firefox 33+.

ref:
https://bugzilla.mozilla.org/show_bug.cgi?id=908440

https://github.com/luckyrat/KeeFox/commit/4bf7c14be9f628b6935b5ef943f6acbe79384966
https://github.com/ithinc/tabutils/blob/423eed5542ef7d3725f08185d3071e60fe6055b2/chrome/content/tabutils-pt.js#L61
Fix urlbar enter hook for Fx33+.

Since the http://hg.mozilla.org/mozilla-central/diff/4ce737d7339d/browser/base/content/urlbarBindings.xml.

Untested for other versions.
Merge remote-tracking branch 'yfdyh000/fix-33'

(pull request ithinc/tabutils#108)

Also update README file about this merge.