New feature to add in support to exclude scala-type mixins from the cove...

Pull request to add in support to exclude the types of mixins that the scala compiler emits in bytecode from coverage reports (Issue #139). The change is pretty non-invasive: there is a new flag added to the jacoco:report goal called "excludeMixins". If it is not set, then it defaults to "false", and the behavior of the report goal is identical to its current behavior. If this flag is configured to "true", then any IMethodCoverages that are mixed-in from a scala trait are ignored in the coverage report. These mixed-in methods can be identified because their line numbers are identical to the line number for a constructor of the class that mixes them in.
I have also included 2 integration tests to verify that this flag behaves as indicated for true and false values of the excludeMixins flag.
Thanks for the great work you are doing on jacoco!
---Tim---