Added an empty unit test project for SamaxLibrary

The target framework of the unit test project is .NET Framework 4.5, so it
cannot be opened in Visual Studio 2010.

Also, the version of the solution file format was updated from 11.00 to
12.00. Visual Studio 2010 SP1 still seems to be able to the solution
though.
Target framework of SamaxLibrary changed to 4.5

The target framework of the SamaxLibrary project was changed from .NET
Framework 4.0 to .NET Framework 4.5 for consistency.
Set up the unit test project with NUnit

The NUnit framework assembly is required to compile the unit test project.
This should be managed automatically by NuGet when NuGet Package Restore
is enabled (right-click on the solution in the Solution Explorer and click
"Enable NuGet Package Restore" to enable it).

To run NUnit tests in Visual Studio 2012, the NUnit Test Adapter is
required. It is available as an extension to Visual Studio 2012, though
not to the express edition ("Extensions and Updates" in the Tools menu).
The test adapter is also available as a NuGet package, which would be
preferable, but the package did not seem to work on Visual Studio 2012
Express (Update 3), so it is not included in the repository yet.