Make the default separator - instead of _.

Our code, as well as rfc4646.txt, says that langauge code subparts
should be separated by -.  (We actually do lang-COUNTRY_region, but
should probably change to follow the rfc.  Luckily, we don't have any
regions yet and are unlikely to ever.)

When then, does babel default to _?  I have no idea.  You can override
that if you call Locale() directly, but dates.py/etc call Locale() for
you, and there's no overriding the separator there.  We just do it in
code.

Auditor: chris

Test plan:
python
>>> import third_party.babel as b
>>> b.parse_locale('pt-BR')
('pt', 'BR', None, None)
>> b.parse_locale('pt_BR')
<ValueError>