Refactoring of all approvers and Approvals entry point #26

The Approvals object and all the Approver objects are to be refactored.

Here are some thoughts after reading again the source code:

• Documentation should be updated to state that Approvals is an easy entry point for accessing only default configuration,
• Approvers can be used for advanced configuration of your tests and validations,
• The custom extension should disappear. Using the csv extension might sound appealing (because of opening the files in other editors), but it actually is really confusing and might lead to other issues (ignoring csv in SCMs, etc.) - I can elaborate on that topic, but my opinion is: we can format the file as if it is a csv file, but it needs to remain a .approved file,
• The custom file name isn't a feature we planned yet, and there is more to think about it than just an entry in the builder object, so it should disappear as well (I'm in favor of an agile approach for that: we implement if and only if we need it (on a real use case))
• As far as possible, the source code itself should allow to understand that Approvers are actually just advanced entry points for approvals. Maybe putting them in an advanced package could do the trick?
• Global refactoring (variable names, functions, etc.)
• Javadoc the whole thing
• Review the error messages and align them to all the other error messages
• Remove from the ApprovalFiles class all the things linked to the extensions