* Added some code from legacy branch, Deedle and Hopac dependencies

TODO
* Refactor, rethink, clean the added code and make it compile and pass tests
* Add all old collections tests and make a benchmark that runs all structures (for not to comment/uncomment anymore)
* Implement IReadOnlyCollection<T> interface for read-only maps/series
* Fix FSharpx dependencies. Just copy relevant code, avoid dependencies when possible

For Period
* Finish specification pdf and do tests with FsCheck with many random dates: Date(Period(date) = date