ArquillianDescriptor should support configuration of extensions

ArquillianDescriptor should provide a way how to configure extensions. This should be done in the same manner as for containers, protocols and groups.

E.g.

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<arquillian>
<extension qualifier="selenium">
<configuration>
<property name="speed">5000</property>
</configuration>
</extension>
</arquillian>