Password field cleanup.

* Ensure "hide" text is centered
* Simplify show/hide button css selectors to be more compatible with IE8
* Fix underflow for browsers other than Firefox 29.

fixes #341
issue #305
Work around the Firefox 29 password flow under the show/hide button problem.

* Set a background color on the show/hide button and position the button just right to cover up the password field.

see https://bugzilla.mozilla.org/show_bug.cgi?id=965237

fixes #349
Merge pull request #373 from mozilla/issues-341-349-password-field-updates

Issues 341 349 password field updates