Changed the semantic of the look_at method.
Changed the semantic of the look_at method.

Before, the `look_at` method aligned the `z` axis with the look_at direction.
This will be problematic in the future if we want to generalize that on n-dimension (the z axis
could not axis at all when n < 3).
Now, the `look_at` method aligns the `x` axis with the look_at direction. Thus we have a chance for
it to be generalizable on n-dimension, `n >= 1`.

jaredly/nalgebra,tomaka/nalgebra,sebcrozet/nalgebra,ProtectedMode/nalgebra,mbStavola/nalgebra,tikue/nalgebra,sebcrozet/nalgebra,tempbottle/nalgebra