Remove redundant debug statement
Mendeley test document added
Add better comment documentation on the way we drop addins
New addin handler that defines behaviour for all unknown addins

The "OtherHandler" class will look for all "addins" that it doesn't understand and (at present) will drop them.
Add option to determine whether unknown addins are dropped
Add extra options to test settings XML files
Fix list check to assign zero index to self

Bug from previous commit