Bug 929172 - gaia unit tests broken in b2g desktop due to fast CSP enhancement r=yurenju

* registers special /common and /shared directories
* changes httpd.js to handle correctly registered directories
* changes the test-agent boilerplate to always use the test's hostname
* add a NO_LOCK_SCREEN env variable for disabling the lockscreen at build time
* add a "unit tests in b2g" travis job