Create a new API for calling Java toolchains.

* Create a new sbt.compiler.javac package
* Create new interfaces to control running `javac` and `javadoc` whether forked or local.
* Ensure new interfaces make use of `xsbti.Reporter`.
* Create new method on `xsbti.compiler.JavaCompiler` which takes a `xsbti.Reporter`
* Create a new mechanism to parse (more accurately) Warnings + Errors, to distinguish the two.
* Ensure older xsbti.Compiler implementations still succeed via catcing NoSuchMethodError.
* Feed new toolchain through sbt.actions.Compiler API via dirty hackery until we can break things in sbt 1.0
* Added a set of unit tests for parsing errors from Javac/Javadoc
* Added a new integration test for hidden compilerReporter key, including testing threading of javac reports.

Fixes #875, Fixes #1542,  Related #1178 could be looked into/cleaned up.


Review by @gkossakowski and @eed3si9n 

cc> @jroper + @havocp (original reporters)
cc> @pvlugter for a quick "zinc" related look-over.
k. my point about TODOs I don't understand is - maybe elaborate enough in the comment that future-us can figure it out ...