Merge pull request #21 from tpetricek/master

Some work on hierarchical indexing, Frame.FromRowKeys, etc.
Merge pull request #22 from tpetricek/master

Fix behaviour of Series.diff function
Merge pull request #23 from tpetricek/master

Hierarchical indexing now uses tuples
Make series.ordered lazy, check if keys are comparable first
Merge branch 'master' of github.com:BlueMountainCapital/FSharp.DataFrame