Use consistent indentation in CSV provider code
Move handling of nulls in CSV to the CSV type generator

To make the generated CSV types nicer (e.g. int?), the previous version of the code added some special handling of `null` values in `StructureInference.fs` (so that a `null` value made a record field optional). This checkin keeps the end-user functionality, but removes special treatment of `null` values from `StructureInference.fs` and instead implements the functionality in `CsvGenerator.fs` (where Heterogeneous types are turned into the desired types like `int?` or `float` with `NaN` values).
Merge branch 'master' of github.com:tpetricek/FSharp.Data

Conflicts:
	src/CsvProvider/CsvInference.fs
Update the Json type provider sample.