Convert Deedle frames/series <-> Math.NET Numerics matrices/vectors (implements #22)

Instead of extension methods it extends a few modules of both Deedle (Frame, Series) and Math.NET Numerics (Vector, DenseVector, SparseVector, Matrix, DenseMatrix, SparseMatrix).

Create a diagonal matrix from a series:
```
words
|> Series.sort
|> Series.rev
|> Series.take 6
|> Series.toVector
|> DenseMatrix.ofDiag
```

Create a matrix from a frame - and back
```
frame [ "Words" => words ]
|> Frame.toMatrix
|> Frame.ofMatrix
```
Or equivalent, although this time forcing a sparse matrix:
```
frame [ "Words" => words ]
|> SparseMatrix.ofFrame
|> Matrix.toFrame
```

Does this come close to what we want here?
Should we drop the redundant implementation?