Make FAQ translatable

Use same technique as WordPress core does with its help tabs.

Also, I noticed that `HMBKP_PLUGIN_PATH` already has a trailing slash
See http://codex.wordpress.org/Function_Reference/plugin_dir_path

Need to update the grunt file after merging, because the format of `faq.php` is different
```
MATCH
.*<strong>.*__\( '(.*)', '\w+' \).*<\/strong>.*

REPLACE
**$1**

MATCH
'(?:<ul>)?<li>' \. __\( '(.*)', 'hmbkp' \) . '<\/li>'?.*

REPLACE
* $1
```