Made BasePlugin.project not static, hopefully fixes #16

This hopefully fixes #16, i've changed BasePlugin.project to be not-static. The only place where it was used was ArtifactSpec where i added a field for the project and a parameter to all the constructors.

Tested this successfully on my (unreleased yet) mod, the build worked fine.

There's a compiled version  with this fix on http://jenkins.bdew.net/maven/net/minecraftforge/gradle/ForgeGradle/1.0-SNAPSHOT/ if anyone wants to test it or something.