Change plugin id to info.solidsoft.pitest

To be compliant with Gradle plugin portal. Small tests refactoring.
Add simple functional tests with nebula-test
Move functional tests to separate source set

To separate unit/integration and functional tests and not run them
on every `gradle check`.
Add task to generate aggregated test report
Move funcTest resources to test sourceSet to workaround bug in idea

Additional resource directories defined in Gradle/Idea are omitted
on build in Idea.
Move nebula-test dependency to testCompile to workaround a bug in Idea

https://youtrack.jetbrains.com/issue/IDEA-121728
Add missing test for targetClasses feature
Add simple test for overriding configuration parameter from command line
#20 Make it easier to manually override PIT dependencies

The plugin adds it own dependencies to already existing and potentially
non empty buildscript configuration "pitest" which can be used to
override PIT dependencies like old TestNG version.