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">libxml</lib>
		<lib max-version="2.0">intl</lib>
		<lib>curl</lib>
		<os>Linux</os>
		<owncloud min-version="6.0.4" max-version="8"/>
	</dependencies>

> So aparently it's not easily possible. but we could also depend on the composer from packagist and get the Version class from there. Should be more failsafe?

well - I was thinking about depending on composer as well - but it will add a hell of code we simply don't need.