added support for global Properties

For my https://github.com/Pheromone/phpcs-security-audit/ rules I need quite a bunch of properties to be set and I was tired of copying the same thing over and over, so I did this patch.

Principally it allows me to set properties on the ruleset that will be applied to any rules.
```xml
<ruleset name="rulesetname">
<properties>
        <property name="prop1" value="val1"/>
        <property name="prop2" value="val2"/>
</properties>
</ruleset>
```
The rules can still override a global property by setting it as usual.