Support .NET 4

* All: Added project configuration.
* Metadata: Added assertions against nullptr because the some API became returning null value.
* Metadata: Whenever possible, change the types that are represented on the signature of methods to version-free.
* Profiling: Added the default implementation for ICorProfilerCallback, ICorProfilerCallback2, ICorProfilerCallback3, ICorProfilerCallback4 and ICorProfilerCallback5.