Add logarithmic (10-base) ticks plugin

My attempt at a logarithmic scales/ticks plugin as mentioned in #105.
I mainly have an interest in 10-base logarithmic scales, so that is what is supported.

The plugin generates nice look logarithmic ticks, e.g. [10⁻¹, 10⁰, 10¹, 10², 10³].
If there are not to many ticks it also creates some in between: [10⁰, 5x10⁰, 10¹, 5x10¹, 10², 5x10², 10³].
If the data to be plotted is all between two powers of ten the currently default tick generator and formatter are used instead.

Mainly tested in latest versions of Safari, Firefox and Chrome.
Possibly still buggy for special cases.
If there are 0 or negative values in the data (`axis.datamin`) the axis.min is set to 0.6 and the ticks start at 10⁰.