Change early load to look for ReactiveUI assemblies based on assembly references

The assembly location check in eager loading of ReactiveUI assemblies, introduced in commit 801fdab4162769a4dc88652b35391a9409c2aa06 to solve issue [#91](https://github.com/reactiveui/ReactiveUI/issues/91), fails when shadow copy is used, e.g. in VSTO add-ons. I created a modified version of the eager loader, based on assembly reference checks & assembly names, thus removing the need for the path check. Tested with ReactiveUI.Test (120/120 tests passed with the 2 tests that my commit adds).