Added support for explicit interface implentations

I added support for explicit interface implementations in .NET interop.

After the normal resolution (which looks at public members) the new code searches for explicit implementations:

* properties: only if a property of a given name is implemented through one interface the explicit property implementation is used
* methods: all explicit method implementations are combined using the existing `MethodInfoFunctionInstance` class
* index properties: only if index properties are implemented through one interface the explicit index property implementation is used

Please have a look at my implementation and consider merging it into your official Jint version.