Fix for https://github.com/scoverage/maven-scoverage-plugin/issues/1

First off, thanks for this awesome plugin. It made my day! :smile:

# Description of the problem
## Steps to reproduce
Set the ``-P:scoverage:dataDir:`` arg to ``/tmp``
Run ``mvn test`` followed by ``mvn scoverage:report``

## Expected Behaviour
* Code coverage reports generated

## Actual behaviour:
 * the Mojo fails because it looks for the measurement and coverage files in ``project.getBuild.getOutputDirectory``

# Details of Fix 

- Change ReportMojo.scala to read the coverage and measurement files from the property ``coverage.data.dir``. It defaults to using the project build output directory as was previously the case.

- Update README to suggest placing configuration for scoverage plugin inside the scala-maven-compiler execution that actually instruments the code. Putting the args in the global plugin configuration results in a Bad Option error in Maven
- Bump version number in pom.xml to 0.95.4

Note: From a look at your code base, I understand you're trying to keep the versions of the scalac-scoverage-plugin and the maven-scoverage-plugin in sync. While I have updated the version of maven-scoverage-plugin in this PR, I would be happy to do the same in the scalac-scoverage-plugin if you'd like me to.