Merge pull request #181 from tpetricek/mainline

Support merging of N series and speedup alignment functions (cf. #179)
Merge pull request #193 from tpetricek/mainline

Merge changes from pull request #186
Merge remote-tracking branch 'tomas/mainline' into mainline

* tomas/mainline:
  Do more renaming ("series" and "column") (c.f. #189)
  WIP: More cleanup - Series and Stats naming and documentation
  Merge series/column renaming changes
  Adding Stats.levelFoo functions (close #185)
  Improve Frame.stack and correctly infer type of values (fix #192)
  Fix documentation for expanding functions according to https://github.com/BlueMountainCapital/Deedle/issues/185#issuecomment-39250612
  Remove test code from Deedle.fsx and minor documentation tweak
  #189 introduced row/col naming uniformity in frame API
Start correcting tests and renames for #196
Speedup df.AppendN (and rename appending functions on frame to merging)

There was a failing statistics test, but this was due to floating point
precision issue, so I updated the test.

The last failing test is the one comparing our skewness with Math.NET
but that is a Math.NET issue (#199)

I have not yet renamed Combine/CombineN in vector construction. I would
like to get rid of Combine and have just a single Merge, but I want to
see if this has any performance implications for two-element mergings
(which is probably the common case).