Add :any-link (Level 4), :link and :visited selectors.

All links are considered "unvisited" for now.
Remove unneeded clones.
auto merge of #1202 : SimonSapin/servo/link-selectors, r=jdm

All links are considered "unvisited" for now.

In particular, this fixes the UA stylesheet rule that underlines links.
auto merge of #1204 : jdm/servo/clones, r=kmcallister

r? @kmcallister
Introduce jsid_to_str to simplify jsval_to_str callers that have an id.
Simplify InterfaceObjectToString.
Rewrite jsval_to_str and introduce jsval_to_domstring.
Add some basic debug info for dumping flows.
auto merge of #1212 : larsbergstrom/servo/dump_flows, r=brson

With @ryanhc

Will add more debugging information for RenderBoxes in InlineFlows and `dump_str` for each of the RenderBoxes later.
auto merge of #1206 : Ms2ger/servo/strings, r=jdm

I'll probably do a followup to actually pass non-nullable strings to the native functions.
fix #1211: for table-related elements, handle temporarily as 'block' display intead of 'inline' display
auto merge of #1216 : june0cho/servo/temp_fix_tableflow, r=kmcallister

In order to fix the wikipedia error, I temporarily change the display property of table-related elements to 'block' instead of 'inline'.
When table is implemented, it should be changed to proper value.

@sonwow
@brson
Initial support for Attr and namespaces.
Modify #1203 jdm's code(Initial support for Attr and namespaces.)