Refactored FilePreProcessor.cs

* Fixed `README.md`
* Refactored `FilePreProcessor.cs`
* Renamed `_fileSystem` to `FileSystem` to reflect protected nature.
* Removed by ref on `ParseFile`.
* Removed 3 arguments on `ParseFile`.
* Made `ParseFile` static.
* Added `ParseResult` class to hold parse results.
* Added `GetFilePathFromLine`
* Added `GetFileContentFromPath`
* Refactored `FileProcessorTests.cs`
* Changed unit tests to reflect `GetFileContentFromPath`


All tests are passing. If there are any other testes that should be written I will be happy to do it.