Consider using client profile as lowest common denominator

[gsnoff]

Currently the .csproj files are targeting the full version of .NET framework 4.0. This causes the libraries to be unusable in projects targeting the client profile, and by extension, systems which only have the client profile installed.

Consider doing the following:

• [ ] Check whether the projects are using any of the standard .NET references not included in client profile (see this).
• [ ] Replace <TargetFrameworkProfile /> tags inside .csproj files with <TargetFrameworkProfile>Client</TargetFrameworkProfile>.
• [ ] Run tests on a Windows system with just .NET 4.0 client profile (but not the full framework) installed.