Fix HTMLImageElement setters.

These would crash because the functions they call assert that they receive
lower-case names.

Fixing the crash allows reflection-embedded.html to finish successfully.
make Node.type_id field private
Add prerequisites for Arch Linux to the readme.
Merge pull request #3222 from Ms2ger/img-reflection

Fix HTMLImageElement setters; r=jdm
Merge pull request #3224 from duncankl/readme-arch-linux-setup

Add prerequisites for Arch Linux to the readme; r=Manishearth
Add a note about autoconf 2.13.

Many people seem to have this question.
Merge pull request #3225 from Ms2ger/note-autoconf

Add a note about autoconf 2.13; r=Manishearth
Disable intermittently failing Document-characterSet-normalization.html (fixes #3209).

As we currently don't support anything but utf-8, this is not a particularly
useful test to run.
Merge pull request #3210 from seanmonstar/private-node-type-id

make Node.type_id field private, access through type_id()
Merge pull request #3226 from Ms2ger/3209-disable

Disable intermittently failing Document-characterSet-normalization.html (fixes #3209); r=jdm
Cargoify servo
Run mach with Python 2.7 if it is present