Move inline stylesheet parsing out of HTML parser.

Instead, use shared code to parse stylesheet content when it is inserted,
whether during parsing or dynamically by script.

Based on work by sanools in #1350.
auto merge of #1984 : mbrubeck/servo/1959-parser-style, r=jdm

This is a partial fix for #1959.  This commit only addresses inline `<style>` elements.  Next I will need to move `<link rel="stylesheet">` handling into shared code, but I wanted to get some early feedback on this piece first.
Move child_inserted into VirtualMethods.

Follow-up from #1984.
auto merge of #2106 : mbrubeck/servo/1959-vtable-followup, r=jdm

Follow-up from #1984. r? @jdm
Implement very basic canvas rendering context logic. No visual display present.