Addresses #185, cleaning up the Deedle statistics API.

* Deprecates statistics functions living on Series
* Adds stats functions for operating on Frame columns
* Fixes bug in expanding stats involving Seq.scan
GH #185, moved other statistics functions to separate module
Additional work on #185, mostly cosmetic refactorings

- move statistics functions out of SeriesModule
- remove statLevel
- combine applyLevel, flatten
- add applyLevelOptional
- add flatMapValues to Series
Small naming change, fixed project file for RProvider plugin.
Additional API cleanup per #185 - remove unnecessary applyLevel/flatten from frame, add NestBy to extensions
Fix NestBy extension signature
Separate out flatMapValues to flatten, add extension method
Turns out we need a Frame.applyLevel after all
Add a convenience non-generic float sum function to Stats
Cleaning up statistics functions etc.

 * Remove dead code in Common.fs and also delete old file Functions.fs
 * Implement our own version of Median and remove Math.NET dependency
   (Math.NET is still great and we should use it for more advanced things,
   but this is better done through extensions in FsLab)
 * Change "Stats" to a type with overloaded methods (details explained
   in https://github.com/BlueMountainCapital/Deedle/issues/185)
Incorporating some changes mentioned in discussion #188/#185:

 - NumSum vs Sum
 - Added ValuesAll and ObservationsAll
 - Fixed TryGetObservation semantics
 - Added maskMissingFrom functionality
Removed dead tests
Fixed quickSelectInPlace name resolution
Attempt changes I mention in #189; testing how they affect our code base
Per #189, this seems to work best with our code base.
Fix fsproj references (for #189)
Opting for a little more flexibility in extracting typed columns per #189
Fixed spelling typo
Merge changes
Merge pull request #193 from tpetricek/mainline

Merge changes from pull request #186