Solve error on checking completely empty files

Fixes #57
Merge pull request #58 from sschaef/t57-error-on-empty-file

Solve error on checking completely empty files
Update BlockImportChecker

Before, the block import checker did mark imports of the form

  import foo.bar.{baz => _, _}

and

  import foo.bar.{baz => buz, _}

as an error, which is wrong because there is no other way to write
such imports in Scala.
Merge pull request #59 from sschaef/block-import-checker-update

Update BlockImportChecker