This pull request makes several fixes to the debug module, and re-enables debug tap unit testing. All tests pass!

The fixes:

1. Merged Fixed : call debug.getmetatable maybe cause NullReferenceException #173 to fix one issue with debug.getmetatable. Unfortunately, github does not support pull request dependencies, so I had to merge this before I went to work.
2. Fixed a potential NullReferenceException when debug.getmetatable was called on a Lua type that COULD have a metatable assigned, but did not. For instance, the following code would throw an NRE:

local m = debug.getmetatable(true)
print(m)

3. debug.setuservalue had a typo that caused it to always throw a ScriptRuntimeException regardless of arguments being correct or not.
4. debug.setuservalue was returning a table, when the standard says it should return the userdata argument passed to it.
5. Re-enabled debug unit tests, with unsupported debug functions commented out.