Indicate password strength using strengthify

 * uses zxcvbn
 * outsourced to separate jquery plugin
 * async loading
 * hide strength meter if input is empty
 * feedback if user enters weakest password
 * fade in and out
 * show tipsy with strength
 * Opera 12, IE8-10, FF 23, Chromium 29
Merge pull request #3234 from owncloud/mjob-password-strength

Indicate password strength