Removed annoying spaces from Scala Console

The existing Scala Console places an annoying space before the first line of the text editor window:

![screen shot 2013-09-22 at 10 47 16 pm](https://f.cloud.github.com/assets/934140/1189462/aed62480-2413-11e3-9996-8d9a93ca0129.png)

This space is extremely ugly. Note also the double space after the `scala>` prompt. These are a result of a [hardcoded hack in the IntelliJ platform](https://github.com/JetBrains/intellij-community/blob/master/platform/lang-impl/src/com/intellij/execution/console/LanguageConsoleImpl.java#L330), which inserts a space if the prompt doesn't end with one (or is empty).

This changes the prompt string to `null` instead of `""`, which provides us the correct behavior. No strange indents inside the rectangular text entry area, and a single space after the `scala>` prompt

![screen shot 2013-09-22 at 10 46 16 pm](https://f.cloud.github.com/assets/934140/1189461/ac44cd48-2413-11e3-84be-75e7bbf48d9b.png)