Make it easier to specify index column & drop it when indexing

After a discussion with @hmansell and @perryvais, they finally convinced
me that `indexRows` family of operations should remove the column that
was used as the key, so this implements this (breaking) change. Using
method-based syntax, you can still call `df.IndexRows("Key", keepColumn=true)`
if you want to keep the column, but F# functions always drop the column.

Aside from that, I also added optional parameter to `ReadCsv` and to
`ofRecords`, so you can now specify index column when creating frames:

    Frame.ofRecords<DateTime>(seq, indexCol="Date")
	Frame.ReadCsv<DateTime>("file.csv", indexCol="Date")