Use thin space (U+2009) as default group separator.

As per official SI recommendation for units and their values.
See http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf p. 133..
Update testcases to match thin space and nbsp usage.

This changes the test_str() method to test for either
str() or unicode() compatibility - this test would've
failed before if we tested this with a currency that
has a unicode currency symbol (e.g. Pakistani Rupees).