Emulate Ruby Sass' url() parsing semantics

[xzyfer]

We've had countless bugs and regressions with parsing url(). This
patch is complete refactor of our url() parsing semantics to closer
match that of Ruby Sass.

TODO

• implement the correct handling of interpolants in url()
• implement the correct semantics for url-prefix()
• determine / implement the correct semantics for @import url()
• replace uri functions / variables with their real_ prefixed versions

Update: I've reduced the scope of this PR to the regression at hand. This update to url() semantics will need to applied to url-prefix() (#1596) and @import url() (#1597) for 3.4.

Fixes #674
Spec sass/sass-spec#539

[xzyfer]

The spec currently failing is with regards to insignificant whitespace. Not entirely sure how to handle this yet.

a {
  b: url(data:image/png;base64,iVI= );
}

Ruby Sass

a {
  b: url(data:image/png;base64,iVI= );
}

LibSass

a {
  b: url(data:image/png;base64,iVI=); // no whitespace
}

[mgreter]

Just retested and latest ruby sass seems to give:

a {
  b: url(data:image/png;base64,iVI=);
}

So this might be fixed already!?

[xzyfer]

This was fixed in 3.3.

replace uri functions / variables with their real_ prefixed versions

Is fixed in 3.3.3.

[xzyfer]

determine / implement the correct semantics for @import url()

Is fixed in 3.3.3.