Force install when addin is upgraded

This will update the value of U_Installed to 'N' when an addin is updated,
forcing a new install on next reboot.

Fixes #11
Display AddIn Description on Admin Form

Addin Description was already saved on database, but now shown on
userForm. Not it is displayed.

Fixes #1
AddInManager/MicroCore: Properly handle reboot

On AddinManager, the collection used to iterate over all addins were being
changed during this iteration. Now it iterates over a copy.

On MicroCore, when an reboot happens, it was registering the
AppEventHandler multiple times.

Fixes #10
AssemblyManager: Properly handle AddinAttribute on Framework update.

On Framework update, the AddInAttribute should be loaded dynamically so we
use the correct type (from byte[] not from running Framework).