Add trace points into gesture library

Add some trace points into gestures library. Now you can use
X property, "Tracing Enabled" to enable and disable tracing
function by setting 1 or 0 respectively.

In order to make the codes more structure and easier to test,
we create another class, Tracer, which is responsible for
controlling tracing.
Besides, we need to modify all the constructors of the subclasses
of Interpreter because we want to link all the interpreters for
one touchpad with one Tracer object. Thus, we also need to modify
the unittests.

BUG=chromium-os:33373
TEST=echo 1 into tracing_on, and set 1 to X property "Tracing Enabled".
Put you finger on the touchpad, and you can see the logs in trace
file.

Change-Id: I41d979c0cd127678e00d19d5bb696213f68015d0
Reviewed-on: https://gerrit.chromium.org/gerrit/30523
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Ready: KuanYu Chu <chuwfuny@chromium.org>
Tested-by: KuanYu Chu <chuwfuny@chromium.org>