Fix @output nsedoc for targets-asn
Let nsedebug.tostr support other datatypes with tostring

Instead of erroring, just use tostring to handle functions, userdata,
etc. This works for booleans and numbers, too, so no need for special
handling there.

Also switched from a string-concatenation model to a table-building one,
with a final concatenation. This could prevent catastrophic slowness
with representing large tables due to continuous reallocation of
strings.
New unittest library and driver script for NSE testing

Please read the documentation. This is a way to add unit testing to NSE
libraries (not scripts, yet). Please add tests to your libraries!
Examples to come in further commits.
Add some unit tests to ipOps. Far from complete coverage!
Fix some requires in unittest.lua