Fix crash during finalization when R.DLL cannot be found.   If that happens,
REngine constructor throws an exception, but the finalizer still runs on
the SafeHandle, which invokes Dispose().   This causes an exception when
removing the non-existing ID from the instances dictionary.
Also, the documentation says that you should not access managed resources
during Finalize(), so I only do if called from Dispose().
Use PreserveObject/ReleaseObject to ensure SEXPREC's are kept alive
by R.
Another stab at making tear-down work properly.
Merge.
Add an option to adapt the initialisation of REngine when the R library is
already loaded in the main application. Basically, this is a merge of the rClr
branch.
Backed out changeset: b19e40d72214

'setupManiLoop' does not work fine because instances returned by REngine.Initialize have one handle in common.
This can be confirmed by calling DangerousGetHandle for each instance.
As the result, for example, 'x' defined by 'engine1' is be able to be referred by 'engine2'.
Update local repo to rClr branch, then merge the default/tip
Add an option to adapt the initialisation of REngine when the R library is
already loaded in the main application.
commot change of visibility required for rclr for data frames tests
Tab indentation changes only for a few lines, to be consistent with the rest of R.NET
Update version.
Add scripts for release

* Building environment requires '/' file separator.
  The scripts converts '\' file separator into '/'.
Enable NuGet Package Restore.
Device fixation.
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.