Streamline file separator used in ScmFile.getPath()

Currently the different ScmManager methods sometimes return the OS separator and sometimes only the forward slash in the ScmFile (used in the complex return object).

As forward slash is currently returned on all OSes for ScmManager.checkIn(...) all other methods like ScmManager.add(...) and ScmManager.remove(...) should behave the same. Also the javadoc at ScmFile.getPath() should be clarified in this regard.