Proper handling of compiler messages

We used to have a triple (errorStream, messageRenderer, verbose) to represent the error reporting strategy.
 Now we have a single MessageCollector abstraction for this.
 As the MessageCollector abstraction was extracted, the need to CompilerMessageLocation and CompilerMessageSeverity arose, too.

compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Severity.java

@@ -20,7 +20,6 @@ package org.jetbrains.jet.lang.diagnostics;
  * @author abreslav
  */
 public enum Severity {
-    LOGGING,
     INFO,
     ERROR,
     WARNING