Using File.separator instead of '\\'

Using '\\' works on Windows but in Linux/OSX it causes files to be
created as /path/to/file/directory\file.java, where 'directory\file.java'
ends up being the name of the output file instead of just 'file.java'.
Merge pull request #10 from jvtrigueros/os-agnostic-paths

Using File.separator instead of '\\'