Added support for severity

Included in the patch:

* Severity at constraint level
* Possibility to change severity of constraint while including in the group
* Default severity level is _INFO_ - currently maintained in xsd.
* HTML report has additional column for severity and constraints are ordered by severity level
* Maven plugin extended to support severity

Not yet in the patch:
* Sonar plugin modifications to set priority

Maven plugin example:

```xml
<configuration>
	<groups>
		<group>default</group>
	</groups>
	<failOnConstraintViolations>true</failOnConstraintViolations>
	<severity>critical</severity>
</configuration>
```

Rule definition example:

```xml
<group id="default">
	<includeConstraint refId="junit4:AssertionMustProvideMessage">
		<severity>blocker</severity>
	</includeConstraint>
	<includeConstraint refId="cdi:BeansMustNotUseFieldInjection"/>
</group>
```