Create a tutorial for new users of PlasmaPy

[namurphy]

It would be really helpful if we could create a tutorial for people who would like to begin using PlasmaPy. This tutorial should be at a pretty high level and not go into great detail for any particular subpackage, but rather give a broad overview of the functionality that is contained within PlasmaPy.

We will undoubtedly need to update this tutorial for every release, but hopefully we could get something ready for the 0.1.0 release (or perhaps the 0.2.0 release).

Thank you!