Move functionality to FileCacheDriver, strip down manager class

And always show Caching admin submenu

Closes #509

Updated for Travis CI links

Fixed php 5.3 Travis CI fail issue

Updated readme for sites info

Used ref from http://www.question2answer.org/sites.php

Travis CI failing for Q2A v18, issue with php 5.3, see below link for more detail
http://question2answer.org/qa/58152/travis-ci-failing-for-q2a-v18-issue-with-php-5-3

Pointed to correct build status png file.

Updated PHPMailer to v5.2.23 (from v5.2.14)

Fix Facebook login issue #510

@Scott, @pupi1985

This fix is written and verified by http://question2answer.org/qa/user/Dan+D. in below thread http://question2answer.org/qa/56966/facebook-login-stopped-working-its-not-even-working-your-site?show=56992#a56992
also see https://github.com/q2a/question2answer/issues/510

I'm just submitting code changes.

Please check line no 593 for currentUrl. 

Thanks to Q2A user Dan D, and Github users valan, iamdand, cksagherian for this patch.

We now use qa-global.js for all the in-page JS (except for qa-admin.js
which regular visitors don’t need).

Clean up some of the JS code