Digits grouping

Hi,

Based on the discussion in issue #58, I've implemented a digit grouping feature which adds a space between groups. That space is not a character (this is a font property), so it is not copied as such when users copy/paste the numbers. The groups size depends on the radix (4 digits for hex and binary, 3 for octal and decimal), and digits in the fractional and the exponent parts are also grouped.

Maybe the parser could be rewritten using the tokenizer, and a setting for turning this feature on/off might be useful.

There are 2 commits:
  - 22a9bdfd1d2e11de87958b1ec2998bc909df9489 adds support for digits grouping for results only.
  - e1a86167c929c3b07b8e0a81be81207f2d11048d extends this support to all expressions.

Please, consider pulling these commits to the official repository.