Refactored Readme, FilePreProcessor, and FileProcessorTests.cs

Fixed Readme
Refactored FilePreProcessor.cs
Renamed 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.
Add GetFilePathFromLine
Added GetFileContentFromPath
Refactored FileProcessorTests.cs
Changed unit tests to reflect GetFileContentFromPath