Typed access to Rows via an automatically implemented interface

Initial implementation that makes it possible to use frame rows in a more
type safe way than using just `df.Rows` (without waiting for type provider
features in the next version of F#).

This lets the user define an interface (by hand) and then fits the columns
of the frame to the members of the interface (see example in tests).
Optimize the performance of typed row access
Cleanup and comment typed rows access, add documentation & update F# Formatting
Support conversion to compatible property type
Remove forgotten comments
Merge pull request #281 from tpetricek/typed

Typed access to Rows via an automatically implemented interface