Moved ScoverageSbtPlugin to package scoverage to enable usage/import fro...

Hi,

I encountered a problem with ScoverageSbtPlugin object today when I tried to import and use the plugin in my project (i.e. use it from scala code and not in sbt build or plugin files). The problem is that it is impossible to import the project since it does not exist in any package. 

To harmonize the ScoverageSbtPlugin object with the class I moved the object into a package called scoverage. 

I hope you will merge this change back into the project.

regards,
Bård