#55: replace unicode plus or minus sign with ascii-compatible form

[benfitzpatrick]

This closes #55.

@matthewrmshin, please review.

[matthewrmshin]

I am thinking, why not just use a single + sign to force a sign, like in C's sprintf or Python's string formatting operations?

[benfitzpatrick]

That precedent is interesting - I'm still leaning towards a +- because it is
immediately human readable, and won't confuse people who want to work
with negative years.

[benfitzpatrick]

I'm not convinced that this (in itself) is better for users, but I suppose at least
it's a standard.