Natural whitespace handling for SBT configuration parser AKA 'no more blankies'

The idea is to remove limitation with blank lines in sbt file.

For backward compatibility only one method is modified -  EvaluateConfigurations.splitExpressions.
Most pull request contains test and sbt files.
The father of idea is  @gkossakowski, solution was implemented by @WarsawScala members in project https://github.com/WarsawScala/sbtchallenge
I'm sorry, I missed the comment. The Scala tokenizer is actually called Scanner, so again it is my bad :)

Have a look at [Scanners.skipNestedComments](https://github.com/scala/scala/blob/2.11.x/src/compiler/scala/tools/nsc/ast/parser/Scanners.scala#L131) to see how they do it. Since you have a compiler around, you can easily get all the tokens from a source file. For example, we do that in the IDE [here](https://github.com/scala-ide/scala-ide/blob/master/org.scala-ide.sdt.core/src/org/scalaide/core/compiler/CompilerApiExtensions.scala#L63)