make pcall loop dependent

[lipp]

fixes #171

[lipp]

@ajacksified Any comments / objections? I'd like to have this in master to continue working with HEAD revs. Most of my "real" tests crash without this fix. #171 has to be investigated further, but this is not trivial and will take some time.

[Tieske]

maybe better to remove pcall from the loop interface. So everywhere just revert to using plain pcall and xpcall. And only in the busted.loop.copas file, upon requiring, replace the global pcall and coxpcall.

If it stays in the interface then I would prefer it to provide general methods loop.initialize() and loop.terminate(), which would then for the copas one do the global pcall substitution en reverting it.

[ajacksified]

I don't like the idea of replacing globals - let's merge this for now to get things moving, since it doesn't effect the external interface, and continue to look for a better solution.