Print unified diff for failed tests and warnings.

It is hard to find what differs in failed tests. I made some quick diffing.

Before:

```bash
[retro@retro  sassc (master=)]❤ make test
true
cd libsass; make
make[1]: Entering directory `/home/retro/work/fun/sassc/libsass'
ar rvs libsass.a context.o functions.o document.o document_parser.o eval_apply.o node.o node_factory.o node_emitters.o prelexer.o sass_interface.o
r - context.o
r - functions.o
r - document.o
r - document_parser.o
r - eval_apply.o
r - node.o
r - node_factory.o
r - node_emitters.o
r - prelexer.o
r - sass_interface.o
make[1]: Leaving directory `/home/retro/work/fun/sassc/libsass'
gcc -O2 -o bin/sassc sassc.o libsass/libsass.a -lstdc++ -lm
ruby spec.rb spec/basic/
Running tests in spec/basic/..

............................F...

31/32 Specs Passed!


-----WARN------
Problem with Ruby compat in spec/basic/16_hex_arithmetic/input.scss
-----WARN------
Problem with Ruby compat in spec/basic/22_colors_with_alpha/input.scss
-----WARN------
Problem with Ruby compat in spec/basic/24_namespace_properties/input.scss

================================
TEST FAILURES!

Failed test spec/basic/28_url

make: *** [test] Error 1

```

After:

```bash
[retro@retro  sassc (test_diff *+$%)]❤ make test
true
cd libsass; make
make[1]: Entering directory `/home/retro/work/fun/sassc/libsass'
ar rvs libsass.a context.o functions.o document.o document_parser.o eval_apply.o node.o node_factory.o node_emitters.o prelexer.o sass_interface.o
r - context.o
r - functions.o
r - document.o
r - document_parser.o
r - eval_apply.o
r - node.o
r - node_factory.o
r - node_emitters.o
r - prelexer.o
r - sass_interface.o
make[1]: Leaving directory `/home/retro/work/fun/sassc/libsass'
gcc -O2 -o bin/sassc sassc.o libsass/libsass.a -lstdc++ -lm
ruby spec.rb spec/basic/
Running tests in spec/basic/..

............................F...

31/32 Specs Passed!


-----WARN------
Problem with Ruby compat in spec/basic/16_hex_arithmetic/input.scss
--- spec/basic/16_hex_arithmetic/sass_output.css	2012-10-03 04:24:32.456655474 +0200
+++ spec/basic/16_hex_arithmetic/sassc_output.css	2012-10-03 04:24:31.888660868 +0200
@@ -6,7 +6,7 @@
   p05: #aabbdd;
   p06: #0101ff;
   p07: blue;
-  p08: cyan;
+  p08: aqua;
   p09: #000000;
   p10: black;
   p11: black;
@@ -15,7 +15,7 @@
   p14: black;
   p15: 10-#222222;
   p16: black;
-  p17: magenta;
+  p17: fuchsia;
   p18: 10 #232323;
   p19: 10/#222222;
   p20: #0b0a0b;

-----WARN------
Problem with Ruby compat in spec/basic/22_colors_with_alpha/input.scss
--- spec/basic/22_colors_with_alpha/sass_output.css	2012-10-03 04:24:39.001593317 +0200
+++ spec/basic/22_colors_with_alpha/sassc_output.css	2012-10-03 04:24:38.420598835 +0200
@@ -4,9 +4,9 @@
   flah: #111111;
   grah: rgba(255, 0, 238, 0.5);
   blah: rgba(1, 2, 3, 0.6);
-  floo: cyan;
+  floo: aqua;
   bloo: rgba(0, 255, 255, 0.7);
-  groo: cyan;
+  groo: aqua;
   hoo: 123;
   moo: 45;
   poo: 6;

-----WARN------
Problem with Ruby compat in spec/basic/24_namespace_properties/input.scss

-----WARN------
Problem with Ruby compat in spec/basic/28_url/input.scss
--- spec/basic/28_url/sass_output.css	2012-10-03 04:24:46.620520963 +0200
+++ spec/basic/28_url/sassc_output.css	2012-10-03 04:24:46.064526242 +0200
@@ -1,18 +1,18 @@
 div {
-  foo: url(bloo/blah.css);
-  bar: url(http://foo/bar/hux.css);
-  foo: url(fudgepop.css);
-  bar: url("http://fudgepop/styles.css");
-  hux: url(http://box_123////fudgepop.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-1.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-2.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-3.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-4.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-5.css);
-  gloo: url("hey3.css");
-  floo: url(hadoop-444.css); }
+  foo: url("bloo/blah.css");
+  bar: url("http://foo/bar/hux.css");
+  foo: url("fudge#{$x}.css");
+  bar: url("http://fudge#{$x}/styles.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  gloo: url("hey#{1+2}.css");
+  floo: url("hadoop-#{$y+321}.css"); }
--- spec/basic/28_url/expected_output.css	2012-10-03 04:14:43.983247029 +0200
+++ spec/basic/28_url/sassc_output.css	2012-10-03 04:24:46.064526242 +0200
@@ -1,18 +1,18 @@
 div {
-  foo: url(bloo/blah.css);
-  bar: url(http://foo/bar/hux.css);
-  foo: url(fudgepop.css);
-  bar: url("http://fudgepop/styles.css");
-  hux: url(http://box_123////fudgepop.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-1.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-2.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-3.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-4.css);
-  hux: url(http://box_123////fudgepop.css);
-  foo: url(http://blah.com/bar-5.css);
-  gloo: url("hey3.css");
-  floo: url(hadoop-444.css); }
+  foo: url("bloo/blah.css");
+  bar: url("http://foo/bar/hux.css");
+  foo: url("fudge#{$x}.css");
+  bar: url("http://fudge#{$x}/styles.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  hux: url("http://box_#{$y}////fudge#{$x}.css");
+  foo: url("http://blah.com/bar-#{$i}.css");
+  gloo: url("hey#{1+2}.css");
+  floo: url("hadoop-#{$y+321}.css"); }

================================
TEST FAILURES!

Failed test spec/basic/28_url

make: *** [test] Error 1
```