Backed out changeset: 572f81d256a2
Enhance debugging views

* SymbolicExpression: shows its internal object type.
* Vector: shows its length and elements.
* Matrix: shows its size and elements.
* DataFrame: shows its size and columns' info.
* InternalString: shows its internal string representation.
Add Factor class.
Replace constant.
Creates a new solution file

* R.NET.sln contains only production projects.
* R.NET.Tests.sln contains all (identical to the old R.NET.sln).
Add GetSlice overloading methods for F#

* Matrix.GetSlice -> Vector will be supported in F# 3.1,
  and tests cannot be compiled without the version.
Update NUnit version.
Rename.
Fix unexpected overwriting of parameter setup.
Add nuspec.
Merge with FSharp.
Fix.
Change Levels property to GetLevels method

* properties should be simple.
Add S4Object class.
Add IsFactor method and some documents.
Add FAKE build script.

* Versions of NuGet packages are fixed by FAKE.
  Need to be fixed.
Add AssemblyInfo and update build script.

* build.fsx: retrieves package version from assembly version.
Change return type for GetFactors

* changes IEnumerable<string> to string array.
Enhance debugging view of DataFrame class.
Fix NA handling of character objects

* The old implementation cannot distinguish NA from "NA".
  As NA represents null object in R,
  it is feasible that the string representation of NA is null.
* DataFrame debugging view follows the change.