Set default locale globally in app

It would be awesome if we could set a default locale for arrow globally. Right now it's hardcoded to `en_us` in several places in the code. If we could set a global default then it would be easier to integrate with packages like Babel and Flask-Babel.

I'm imagining an API like this:

``` python
import arrow
from .myapp.i18n import get_locale

# It can be set statically...

arrow.set_locale_default('sv_se')

# ... or dynamically like this:

def get_arrow_locale():
return get_locale().lower()

arrow.set_locale_default(get_arrow_locale)
```