angular directive for AJAX forms

As title, an angular directive that automates logic for AJAX forms.

Contains two changes to core/Config.php:

- First is bug fix caused by typo.  See commit https://github.com/piwik/piwik/commit/773160a34320b85b7f84b781c69727d8e48dd311
- Second is modification of old default behavior. The bug caused new default behavior which is now relevant and to be kept. See commit https://github.com/piwik/piwik/commit/79496d4882149a2c977ff818a11ed0a8da0a9dac . This change will return an empty array if a config section doesn't exist.

Rationale for second change: Plugins may require configuration options to be set in INI config. They will not be able to modify global.ini.php so when they first access a non-existant config section, an error will throw. This effectively bars plugins from using INI config.