Fix the out-of-bound failure in stylesheet iterator.

Rewrite the iterator as internal instead of external.
This is much easier for recursive tree traversal.
Fix unused import warnings.
auto merge of #1137 : SimonSapin/servo/fix-stylesheet-iterator, r=metajack
auto merge of #1140 : SimonSapin/servo/unused-imports, r=metajack
Optimize reflow by changing enums to traits and inlining more
auto merge of #1132 : pcwalton/servo/optimize-reflow, r=larsbergstrom

r? @metajack
submodules: Update submodules.
submodule: Update rust-http to a fixed version
Add rainbow perf test case