Raise a managed exception if the Class was not found during construction.

Previously the Id constructor might send a null pointer through SendMessage which was harder to debug.
Add support to configure the SDK version (assembly linkage).

This allows compilation using mscorlib v2 for example.
Register all ObjectiveCClass types in an assembly.

Previously only exported (e.g. public) types were registered. However, it is sometimes useful to register non-public Mono types with ObjectiveC for interop purposes while keeping the Mono type hidden from API consumers.
Merge branch 'hotfix/null-class-exception' of https://github.com/spicypixel/monobjc into spicypixel-hotfix/null-class-exception
Merge branch 'feature/mono-sdk-option' of https://github.com/spicypixel/monobjc into spicypixel-feature/mono-sdk-option
Merge branch 'hotfix/register-all-types' of https://github.com/spicypixel/monobjc into spicypixel-hotfix/register-all-types