adding dependencies for command line tools and php libraries

This PR adds dependency management for php extensions, command line tools, operating systems and owncloud version

```xml
	<dependencies>
		<php min-version="5.4" max-version="5.5"/>
		<database min-version="3.0">sqlite</database>
		<database>mysql</database>
		<command os="linux">grep</command>
		<command os="windows">notepad.exe</command>
		<lib min-version="1.2">xml</lib>
		<lib max-version="2.0">intl</lib>
		<lib>curl</lib>
		<os>Linux</os>
		<oc min-version="6.0.4" max-version="8"/>
	</dependencies>

This class could also contain the `checkServer()` and other checks that are now somewhere else.
Something to think about later.