Support customization of master-detail record handling

[mthamil]

Using the fluent layout classes for record configuration, it is almost possible to completely decouple record models from the FlatFile library. The only exception is in the case of master/detail record relationships. This currently requires the use of the interfaces IMasterRecord and IDetailRecord.

In this pull request, I have factored out master-detail record handling into a pluggable mechanism. The stateful side of master-detail tracking can be replaced in its entirety using an implementation of the IMasterDetailTracker interface, or just the checking of whether records are a master or detail and what to do with them can be changed by providing a MasterDetailTrackerBase instance with different constructor parameters.

The changes should be backward compatible since the default implementation preserves the current behavior.

I have added tests using, somewhat ironically, an attribute-based implementation of master-detail records. It is really just based on the existing master-detail tests.

As a side note, I was not able to execute the tests in Visual Studio until I installed the xUnit Visual Studio runner package.

I welcome feedback and hope that this could be merged.