Adding initial version of parser trait

Added parse_chunk method declaration to parser

Removed unnecessary visibilty for parse_chunk function

Implemented parse_chunk function

Implemented parse_chunk fn for ServoHTMLParser

Moved parser trait to mod.rs and added finish fn

added licence header to mod.rs and other review comments

Fixed trailing space issue

Fixed failed tabular space test
layout: Actually calculate restyle damage while styling.

...oops. Looks like I forgot to do this.

r? @pcwalton
auto merge of #3836 : cgaebel/servo/calculate-restyle-damage, r=pcwalton

...oops. Looks like I forgot to do this.

r? @pcwalton
auto merge of #3718 : juzer10/servo/master, r=jdm

We have created parser trait and declared parse_chunk function in this trait. We are yet to implement this parse_chunk for ServoHTMLParser struct.
Infrastructure for synchronous script loading

This implements the parts of the "prepare a script element" algorithm that are
required for synchronous scripts.  It also adds some infrastructure for future
support of the `async` and `defer` attributes.
Switch to synchronous script loading

This removes the old code for asyncronously loading scripts during HTML
parsing and then executing them afterward.

Fixes #3356.
Don't force reflow on content changes

This fixes a performance regression caused by the previous patches.  Once we
allowed script and layout to run during parsing, it was running too often
(every time the document changed and called window.reflow).

Fixes #1269.
Implement cloning steps for script element
Test for sync script loading