Remove #r directives from script and pass them to ScriptEngine instead

This PR contains the following:
 - Refactoring of FilePreProcessor
 - #r directives are removed from script and passed to ScriptEngine instead. Fixes #140
 - `ProcessFile` now returns a `FilePreProcessorResult` which contains
  - Loaded files
  - References
  - Using statements
  - Resulting script (code)

This result object will make f.ex. ScriptCsPad development easier :smile: