allows injection of time fixtures to axis

This could make it much easier to use a different time fixture, whilst retaining backwards compatibility if no fixture is provided.

Related to https://github.com/shutterstock/rickshaw/issues/140#issuecomment-21154192, however the suggestion in the comment would force using a `fixedTimeUnit` that won't get re-calculated if the graph gets updated.

Being able to inject a different time fixture makes it very easy to, e.g. overwrite the `formatTime` or `formatDate` functions and feed those to the axis easily.