Support for unit conversions

[chriseppstein]

This code gives an error:

@base: 12px / 1em;
div { width: 16px / @base; }

When it would generate valid CSS:

div {width: 1.333em;}

Ditto for units that are comparable under addition and subtraction:

div {width: 10cm + 100mm;} /* Should be 20cm or 200mm */