Add test cases for modulo expression.
Add parser support for modulo expression.
Implement modulo support in Number.
Merge pull request #314 from xen0n/modulo-expression

Support for modulo expressions
Fix the fallback behavior of / and - on non-strings.

The default is to treat these like a single unit, so "a" - "b" produces
"a"-"b".  The code was neglecting to use String.unquoted, so the
resulting string would have even more quotes around it, which was...
incorrect.

This mostly had an effect on using CSS syntax like top/1px inside a
variable, where division is evaluated.  The result was quoted, which
changed the semantics.
Fix handling of relative imports again.
Make line numbers in errors moderately less wrong.
Fix parsing of `@if(...)`, where there's no literal space.
Stop using apply_vars on block arguments; use the parser instead.
Avoid u'' for 3.2's sake.
Bump version to 1.3.4.