Properly highlight setter operators taking into account the _= suffix.

Fixes a semantic highlighting problem for setter operators. Before calculating the starting position of the setter operator the _= suffix must be dropped or the starting position of the operator will be two characters too early. An instance of this can be seen in Analyzer.scala. In the following code `value` is a setter operator and it was being highlighted starting with the 'g', 'e', and 'e' respectively.

NOTE: This commit is not compatible with Scala 2.10 or 2.9. I will work on separate PRs for the previous Scala versions.

```scala
  settings.YpresentationDebug.value = presCompLog.isTraceEnabled
  settings.YpresentationVerbose.value = presCompLog.isDebugEnabled
  settings.verbose.value = presCompLog.isDebugEnabled
```