Fix cobertura coverage statistics when filtering packages or targets

[swhitty]

Cobertura files include overall coverage statistics on the root <coverage> element of the file. When packages or targets are excluded via --exclude-packages, --exclude-targets these overall statistics are no longer valid. While it is possible to recalculate using the included <line> elements within the file it is much easier if xcc could do this automatically.

<!-- a single file is included with 100% coverage, due to excludes 50% is reported -->
<coverage line-rate="0.5" lines-covered="50" lines-valid="100">
  <sources>
    <source>/Developer/MyProject</source>
  </sources>
  <packages>
    <package name="Developer.MyProject.Sources.Foo" line-rate="1">
      <classes>
        <class name="Developer.MyProject.Sources.Foo.Bar" line-rate="1">
          <lines>
            <line number="7" hits="6"/>
          </lines>
        </class>
     </classes>
  </package>
</coverage>

This PR recalculates the coverage statistics after any filters have been applied so the resulting cobertura files correctly reflect the statistics of the included files and targets.