App dependencies php version

First app dependencies just arrived: minimum and maximum php versions and supported databases:

```xml
	<dependencies>
		<php>
			<min-version>5.4</min-version>
			<max-version>5.4</max-version>
		</php>
		<database>sqlite</database>
		<database>mysql</database>
	</dependencies>

Could also do an `array_merge(array('public' => array(), 'types' => array(), ...), $array)` to ensure the keys exist?