Base plugin

[breskeby]

I did some cleanup on the Frege Plugin and the creation of compile tasks

• some renamings (e.g. CompileTask -> FregeCompile) for matching naming conventions used by Gradle
• introducing base-frege plugin that wires frege compile task per sourceSet
• frege-lang plugin now just applies java plugin + frege base plugin + common frege tasks
• frege repl and doc task can use some more cleanup;
• I removed some flags from the frege compiler (no in-process compilation)
• more integTest coverage (different gradle version + different frege version)

TODO:

• cleanup frege-gradle-plugin classpath
• more integration testing of mixing java + frege

[Dierk]

Cool, thanks!