emacs: Change the default thousands separator to a space

emacs: Change the default thousands separator to a space

This had been discussed and decided on IRC.

Rationale:
  Therefore the space is recommended in the SI/ISO 31-0 standard, and the
  International Bureau of Weights and Measures states that "for numbers with
  many digits the digits may be divided into groups of three by a thin space, in
  order to facilitate reading. Neither dots nor commas are inserted in the
  spaces between groups of three".

(http://en.wikipedia.org/wiki/Decimal_separator#Digit_grouping)