Scan AppDomain assemblies, do not try loading already loaded assemblies

[mauroservienti]

Under certain circumstances, primarily in Unit Tests, the Assembly Scanner might fail because it tries to load already loaded assemblies. The pull request introduces the following changes:

1. The scanner first scans AppDomain assemblies
2. The scanner now uses a dictionary (key: Assembly full name) to keep track of already loaded assemblies
3. The scanner try/catches FileLoadException exceptions when using Assembly.LoadFrom