Build: Do not swallow Mako template errors in src/components/style/properties.rs.mako
Build: Ugrade Mako to 0.9.1, to support Python 3.4
Build: Use the same Python for Mako as for DOM codegen.

This defaults to Python 2.x and makes the previous commit useless,
but upgrading Mako doesn’t hurt anyway.
auto merge of #1995 : SimonSapin/servo/mako-py34, r=metajack

On a system where `which python` is Python 3.4.x, the build failed with a very unhelpful error message. The root cause is that Mako 0.8.1 was too old.
Store Window.active_timers in a HashMap

A HashMap allows easily looking up a timer and canceling it in
Window.ClearTimeout.

Fixes #1477.