Different approach for error-prone compiler plugin: refactor plexus-compiler-javac to allow easier extension and classloading, and depend directly on error_prone_core from plexus-compiler-javac-errorprone.
Add missing copyright and some notes to clarify the pull request
Merge branch 'master' of https://github.com/alexeagle/plexus-compiler into alexeagle-fix-pr-19

Conflicts:
	plexus-compilers/plexus-compiler-javac-errorprone/pom.xml
	plexus-compilers/plexus-compiler-javac-errorprone/src/main/java/org/codehaus/plexus/compiler/javac/JavacErrorProneCompiler.java
manually merge pr 19