Lots of fixes to gritty details of how Sass treats url().

Basically: if it looks like it's a single bare URL, only interpolations
are allowed.  Otherwise, it's expected to be a whole expression.

Handling of interpolation in barewords is much improved.

Also fixed the wrapping of SassSyntaxError.
Sass allows space after the ! in !important.
Allow CSS escapes in barewords.
Unfix this test, alas.
Oops, broke Url and Function a bit.
Fix interactions between interpolation and function literals.
Update sprite-map test.

The hash is affected by the repr of Sass types, which I changed
recently.
Give up on matching Ruby's quoting behavior for now.
Better approach to registering Compass functions.
Fix @extend heisenbug when extending a selector that appears twice in the same rule.