Changes to support latest rust-layers

Now tiles are stored in TileGrid and layer methods are simple instance
methods.
Update submodule pointer for core-graphics
Respond to review comments
Merge pull request #2843 from mrobinson/tilegrid

Changes to support latest rust-layers
Temporarily disable the 'borders' test

This test has been failing locally on OS X for me for quite some time,
although since #2843 this started affecting the Travis builders as well.
Getting to the root of this problem is tracked by #2848.
Merge pull request #2849 from zwarich/disable-borders-test

Temporarily disable the 'borders' test
Implement MessageEvent.
Added 'parent is/was in tree' param to bind_to_tree/unbind_from_tree

According to a talk with Ms2ger, both bind_to_tree / unbind_from_tree
should be called regardless if the tree is part of a Document. This
information is now passed as a parameter to their respective virtual
methods.
Use tree_in_doc on Style elem to parse its own CSS, if needed
Merge pull request #2852 from Ms2ger/MessageEvent

Implement MessageEvent.
Merge pull request #2835 from brunoabinader/bind_to_tree_fix

Added 'parent is/was in tree' param to bind_to_tree/unbind_from_tree; r=Ms2ger
Merge pull request #2845 from glennw/update-cg

Update submodule pointer for core-graphics