Calculator: guard against empty string as non-result.

Eval of '! + 1' returns an empty string, so we'll make it less likely
for that to happen (by not initializing tmp_result to an empty string)
and guard against it if it still does.

Of course, this shouldn't trigger on '! + 1', so that still needs to be
resolved.
Calculator: remove ! from trigger regex.

There appears to be no value in this beingincluded and it makes things
evaluate incorrectly, so we'll just remove it.
Merge pull request #585 from mwmiller/bangalator

Calculator: fix bangful query result. Ping @jagtalon