Modify signature of FindExtensionPoints and FindExtensions

[CharliePoole]

Modify the signatures of the above so that they return the original instance of the ApplicationManager.

This will permit easier initialization such as

var mgr = new ExtensionManager(PATH)
    .FindExtensionPoints(ASSEMBLY1, ASSEMBLY2...)
    .FindExtensions(START_DIRECTORY);

This should be implemented in a non-breaking manner, changing the return value from void to ExtensionManager but not modifying the arguments in any way.