Updated project to be compatible with VS2010 and .NET 4.0

The projects in the RexPro Client solution would not load in VS 2010. You get an error message about compatibility. 

NuGet produces an error if you try to install using .NET 4.0:
```
Attempting to resolve dependency 'MsgPack.Cli (≥ 0.3)'.
Installing 'MsgPack.Cli 0.3.1'.
Successfully installed 'MsgPack.Cli 0.3.1'.
Installing 'RexProClient 1.0.4864.22416'.
Successfully installed 'RexProClient 1.0.4864.22416'.
Adding 'MsgPack.Cli 0.3.1' to Fabric.Api.Traversal.
'packages.config' already exists. Skipping...
Successfully added 'MsgPack.Cli 0.3.1' to Fabric.Api.Traversal.
Adding 'RexProClient 1.0.4864.22416' to Fabric.Api.Traversal.
Uninstalling 'RexProClient 1.0.4864.22416'.
Successfully uninstalled 'RexProClient 1.0.4864.22416'.
Successfully uninstalled 'MsgPack.Cli 0.3.1'.
Install failed. Rolling back...
Could not install package 'RexProClient 1.0.4864.22416'. You are trying to install this package into a project that targets '.NETFramework,Version=v4.0', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
```

This commit resolves the VS 2010 issue. I'm not sure if it will fix the NuGet issue. All 27 tests pass, so it doesn't seem that using .NET 4.5 was a necessity.