Implements textAlign

resolves #5
I like the `mapping`. I will update it.

About supporting `rtl`:
If I understand `rtl` right.. it means the `canvas`-element gets a `dir`-attribute with `rtl` -value.
If this `dir="rtl"` -value is present. `start` will be start on right-side (`ltr` it's starts on left-side)..
Means mapping `left` to `start` wont be correct. I will need to check how SVG behaves on `rtl`-mode..