Move pitest configuration to rootProject.buildscript

pitest configuration with PIT dependencies created by the plugin
was created as current project dependency which forced an addition
of snapshot repoitory in the project scope when PIT snapshot version
was used. This commit moves it to buildscript scope of the rootProject.
Fix #15 -  pom dependencies are not passed to PIT

This prevents 'error in opening zip file' when PIT tried
to open pom file as jar/zip.
Switch to PIT 1.0.0
Upgrade wrapper to 1.12