Add a ShutdownState type in the compositor

Currently the compositor tracks its state in the processing of shutting
down with two boolean instance variables `shutting_down` and `done`, but
these two variables really implement a three-state abstraction.
Made Page.window_size and Page.next_subpage_id Traceable, added trait Encodable for WindowSizeData, ViewportPx, PagePx, and DevicePixel
Warning police.
Merge pull request #2776 from glennw/warnings

Warning police.
Remove some mem::transmute calls and reduce some unsafe blocks in parallel.rs.
Print a more helpful debug message when creating an element in the HTML parser.

Using {:?} prints a debugging representation such as

  collections::string::String{
    vec: collections::vec::Vec<u8>{
      len: 4u,
      cap: 4u,
      ptr: (0x7f75670285d8 as *mut ())
    }
  }

which is not very helpful.
Don't fail on parsing URLs in the html parser
Merge pull request #2779 from Ms2ger/print

Print a more helpful debug message when creating an element in the HTML parser.
Merge pull request #2777 from Manishearth/try_parse_url

Don't fail on parsing URLs in the html parser
Update to latest rust-geom
Adopt Rect::contains in compositor hit-testing

This does make hit-testing inclusive in the right/bottom edges, whereas it
was only inclusive in the top/left edges before.
Merge pull request #2780 from eshyong/reduce-untraceables

Made Page.window_size and Page.next_subpage_id Traceable, added trait En...
Use "Fedora" instead of "Fedora Core" in README
Merge pull request #2783 from molnarg/master

Use "Fedora" instead of "Fedora Core" in README
Merge pull request #2778 from Ms2ger/parallel

Remove some mem::transmute calls and reduce some unsafe blocks in parallel.rs.
Merge pull request #2767 from zwarich/shutdown-state

Add a ShutdownState type in the compositor
Merge pull request #2781 from zwarich/compositor-contains

Adopt Rect::contains in compositor hit-testing