Unformat bad input when instantiated (as mentioned in issue #53)

First commit is an update to the unformatNumeral negation logic to check for nested brackets and multiple minus signs. This should hopefully suppress any NaN's being returned from code like `numeral().unformat('--1')`
Also I corrected a typo in unformatTime.

Second commit is an update to the top-level parser to unformat non-numerical input when necessary. This doesn't throw an error on un-unformattable (what a word!) input but it will just instantiate to zero in this case as usual.