[runtime] Fix for bug #624914. Runtime has to remap System.Config 2.0 to 4.0

[runtime] Fix for bug #624914. Runtime has to remap System.Config 2.0 to 4.0

The bug was caused by the runtime loading two copies of System.Configuration -
one for the 4.0 profile (loaded by xsp4 itself) and another for the 2.0 profile
(loaded as a reference for BlogEngine.Core.dll) and for that reason, the static
class System.Configuration.ConfigurationManager was instantiated twice in the
BlogEngine application's AppDomain which, in effect, led to the exception since
the configuration system wasn't reset from the default one to the system.web-specific
one. The commit makes runtime remap System.Configuration 2.0 to 4.0.

Thanks to Zoltan Varga for catching the issue.